perldebug - Perl debugging
First of all, have you tried using the -w switch?
If you're new to the Perl debugger, you may prefer to read perldebtut, which is a tutorial introduction to the debugger .
If you invoke Perl with the -d switch, your script runs under the Perl source debugger. This works like an interactive Perl environment, prompting for debugger commands that let you examine source code, set breakpoints, get stack backtraces, change the values of variables, etc. This is so convenient that you often fire up the debugger all by itself just to test out Perl constructs interactively to see what they do. For example:
- $ perl -d -e 42
In Perl, the debugger is not a separate program the way it usually is in the typical compiled environment. Instead, the -d flag tells the compiler to insert source information into the parse trees it's about to hand off to the interpreter. That means your code must first compile correctly for the debugger to work on it. Then when the interpreter starts up, it preloads a special Perl library file containing the debugger.
The program will halt right before the first run-time executable statement (but see below regarding compile-time statements) and ask you to enter a debugger command. Contrary to popular expectations, whenever the debugger halts and shows you a line of code, it always displays the line it's about to execute, rather than the one it has just executed.
Any command not recognized by the debugger is directly executed
(eval'd) as Perl code in the current package. (The debugger
uses the DB package for keeping its own state information.)
Note that the said eval is bound by an implicit scope. As a
result any newly introduced lexical variable or any modified
capture buffer content is lost after the eval. The debugger is a
nice environment to learn Perl, but if you interactively experiment using
material which should be in the same scope, stuff it in one line.
For any text entered at the debugger prompt, leading and trailing whitespace
is first stripped before further processing. If a debugger command
coincides with some function in your own program, merely precede the
function with something that doesn't look like a debugger command, such
as a leading ;
or perhaps a +
, or by wrapping it with parentheses
or braces.
There are several ways to call the debugger:
On the given program identified by program_name
.
Interactively supply an arbitrary expression
using -e
.
Debug a given program via the Devel::Ptkdb
GUI.
Debug a given program using threads (experimental).
The interactive debugger understands the following commands:
Prints out a summary help message
Prints out a help message for the given debugger command.
The special argument of h h
produces the entire help page, which is quite long.
If the output of the h h
command (or any command, for that matter) scrolls
past your screen, precede the command with a leading pipe symbol so
that it's run through your pager, as in
- DB> |h h
You may change the pager which is used via o pager=...
command.
Same as print {$DB::OUT} expr
in the current package. In particular,
because this is just Perl's own print function, this means that nested
data structures and objects are not dumped, unlike with the x
command.
The DB::OUT
filehandle is opened to /dev/tty, regardless of
where STDOUT may be redirected to.
Evaluates its expression in list context and dumps out the result in a
pretty-printed fashion. Nested data structures are printed out
recursively, unlike the real print function in Perl. When dumping
hashes, you'll probably prefer 'x \%h' rather than 'x %h'.
See Dumpvalue if you'd like to do this yourself.
The output format is governed by multiple options described under Configurable Options.
If the maxdepth
is included, it must be a numeral N; the value is
dumped only N levels deep, as if the dumpDepth
option had been
temporarily set to N.
Display all (or some) variables in package (defaulting to main
)
using a data pretty-printer (hashes show their keys and values so
you see what's what, control characters are made printable, etc.).
Make sure you don't put the type specifier (like $
) there, just
the symbol names, like this:
- V DB filename line
Use ~pattern
and !pattern
for positive and negative regexes.
This is similar to calling the x
command on each applicable var.
Same as V currentpackage [vars]
.
Display all (or some) lexical variables (mnemonic: mY
variables)
in the current scope or level scopes higher. You can limit the
variables that you see with vars which works exactly as it does
for the V
and X
commands. Requires the PadWalker
module
version 0.08 or higher; will warn if this isn't installed. Output
is pretty-printed in the same style as for V
and the format is
controlled by the same options.
Produce a stack backtrace. See below for details on its output.
Single step. Executes until the beginning of another statement, descending into subroutine calls. If an expression is supplied that includes function calls, it too will be single-stepped.
Next. Executes over subroutine calls, until the beginning of the next statement. If an expression is supplied that includes function calls, those functions will be executed with stops before each statement.
Continue until the return from the current subroutine.
Dump the return value if the PrintRet
option is set (default).
Repeat last n
or s command.
Continue, optionally inserting a one-time-only breakpoint at the specified line or subroutine.
List next window of lines.
List incr+1
lines starting at min
.
List lines min
through max
. l -
is synonymous to -
.
List a single line.
List first window of lines from subroutine. subname may be a variable that contains a code reference.
List previous window of lines.
View a few lines of code around the current line.
Return the internal debugger pointer to the line last executed, and print out that line.
Switch to viewing a different file or eval statement. If filename
is not a full pathname found in the values of %INC, it is considered
a regex.
evaled strings (when accessible) are considered to be filenames:
f (eval 7)
and f eval 7\b
access the body of the 7th evaled string
(in the order of execution). The bodies of the currently executed eval
and of evaled strings that define subroutines are saved and thus
accessible.
Search forwards for pattern (a Perl regex); final / is optional. The search is case-insensitive by default.
Search backwards for pattern; final ? is optional. The search is case-insensitive by default.
List (default all) actions, breakpoints and watch expressions
List subroutine names [not] matching the regex.
Toggle trace mode (see also the AutoTrace
option).
Trace through execution of expr
.
See Frame Listing Output Examples in perldebguts for examples.
Sets breakpoint on current line
Set a breakpoint before the given line. If a condition
is specified, it's evaluated each time the statement is reached: a
breakpoint is taken only if the condition is true. Breakpoints may
only be set on lines that begin an executable statement. Conditions
don't use if
:
- b 237 $x > 30
- b 237 ++$count237 < 11
- b 33 /pattern/i
Set a breakpoint before the first line of the named subroutine. subname may be a variable containing a code reference (in this case condition is not supported).
Set a breakpoint at first line of subroutine after it is compiled.
Set a breakpoint before the first executed line of the filename, which should be a full pathname found amongst the %INC values.
Sets a breakpoint before the first statement executed after the specified subroutine is compiled.
Delete a breakpoint from the specified line.
Delete all installed breakpoints.
Set an action to be done before the line is executed. If line is omitted, set an action on the line about to be executed. The sequence of steps taken by the debugger is
- 1. check for a breakpoint at this line
- 2. print the line if necessary (tracing)
- 3. do any actions associated with that line
- 4. prompt user if at a breakpoint or in single-step
- 5. evaluate line
For example, this will print out $foo every time line 53 is passed:
- a 53 print "DB FOUND $foo\n"
Delete an action from the specified line.
Delete all installed actions.
Add a global watch-expression. We hope you know what one of these is, because they're supposed to be obvious.
Delete watch-expression
Delete all watch-expressions.
Display all options
Set each listed Boolean option to the value 1
.
Print out the value of one or more options.
Set the value of one or more options. If the value has internal
whitespace, it should be quoted. For example, you could set o
pager="less -MQeicsNfr"
to call less with those specific options.
You may use either single or double quotes, but if you do, you must
escape any embedded instances of same sort of quote you began with,
as well as any escaping any escapes that immediately precede that
quote but which are not meant to escape the quote itself. In other
words, you follow single-quoting rules irrespective of the quote;
eg: o option='this isn\'t bad'
or o option="She said, \"Isn't
it?\""
.
For historical reasons, the =value
is optional, but defaults to
1 only where it is safe to do so--that is, mostly for Boolean
options. It is always better to assign a specific value using =
.
The option
can be abbreviated, but for clarity probably should
not be. Several options can be set together. See Configurable Options
for a list of these.
List out all pre-prompt Perl command actions.
Set an action (Perl command) to happen before every debugger prompt. A multi-line command may be entered by backslashing the newlines.
Delete all pre-prompt Perl command actions.
Add an action (Perl command) to happen before every debugger prompt. A multi-line command may be entered by backwhacking the newlines.
List out post-prompt Perl command actions.
Set an action (Perl command) to happen after the prompt when you've just given a command to return to executing the script. A multi-line command may be entered by backslashing the newlines (we bet you couldn't have guessed this by now).
Delete all post-prompt Perl command actions.
Adds an action (Perl command) to happen after the prompt when you've just given a command to return to executing the script. A multi-line command may be entered by backslashing the newlines.
List out pre-prompt debugger commands.
Set an action (debugger command) to happen before every debugger prompt. A multi-line command may be entered in the customary fashion.
Because this command is in some senses new, a warning is issued if
you appear to have accidentally entered a block instead. If that's
what you mean to do, write it as with ;{ ... }
or even
do { ... }
.
Delete all pre-prompt debugger commands.
Add an action (debugger command) to happen before every debugger prompt. A multi-line command may be entered, if you can guess how: see above.
Redo a previous command (defaults to the previous command).
Redo number'th previous command.
Redo last command that started with pattern.
See o recallCommand
, too.
Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See
o shellBang
, also. Note that the user's current shell (well,
their $ENV{SHELL}
variable) will be used, which can interfere
with proper interpretation of exit status or signal and coredump
information.
Read and execute debugger commands from file.
file may itself contain source
commands.
Display last n commands. Only commands longer than one character are listed. If number is omitted, list them all.
Quit. ("quit" doesn't work for this, unless you've made an alias)
This is the only supported way to exit the debugger, though typing
exit twice might work.
Set the inhibit_exit
option to 0 if you want to be able to step
off the end the script. You may also need to set $finished to 0
if you want to step through global destruction.
Restart the debugger by exec()ing a new session. We try to maintain
your history across this, but internal settings and command-line options
may be lost.
The following setting are currently preserved: history, breakpoints, actions, debugger options, and the Perl command-line options -w, -I, and -e.
Run the debugger command, piping DB::OUT into your current pager.
Same as |dbcmd but DB::OUT is temporarily selected as well.
Define a command alias, like
- = quit q
or list current aliases.
Execute command as a Perl statement. A trailing semicolon will be supplied. If the Perl statement would otherwise be confused for a Perl debugger, use a leading semicolon, too.
List which methods may be called on the result of the evaluated expression. The expression may evaluated to a reference to a blessed object, or to a package name.
Displays all loaded modules and their versions
Despite its name, this calls your system's default documentation
viewer on the given page, or on the viewer itself if manpage is
omitted. If that viewer is man, the current Config
information
is used to invoke man using the proper MANPATH or -M
manpath option. Failed lookups of the form XXX
that match
known manpages of the form perlXXX will be retried. This lets
you type man debug
or man op
from the debugger.
On systems traditionally bereft of a usable man command, the debugger invokes perldoc. Occasionally this determination is incorrect due to recalcitrant vendors or rather more felicitously, to enterprising users. If you fall into either category, just manually set the $DB::doccmd variable to whatever viewer to view the Perl documentation on your system. This may be set in an rc file, or through direct assignment. We're still waiting for a working example of something along the lines of:
- $DB::doccmd = 'netscape -remote http://something.here/';
The debugger has numerous options settable using the o
command,
either interactively or from the environment or an rc file.
(./.perldb or ~/.perldb under Unix.)
recallCommand
, ShellBang
The characters used to recall command or spawn shell. By
default, both are set to !
, which is unfortunate.
pager
Program to use for output of pager-piped commands (those beginning
with a | character.) By default, $ENV{PAGER}
will be used.
Because the debugger uses your current terminal characteristics
for bold and underlining, if the chosen pager does not pass escape
sequences through unchanged, the output of some debugger commands
will not be readable when sent through the pager.
tkRunning
Run Tk while prompting (with ReadLine).
signalLevel
, warnLevel
, dieLevel
Level of verbosity. By default, the debugger leaves your exceptions and warnings alone, because altering them can break correctly running programs. It will attempt to print a message when uncaught INT, BUS, or SEGV signals arrive. (But see the mention of signals in BUGS below.)
To disable this default safe mode, set these values to something higher
than 0. At a level of 1, you get backtraces upon receiving any kind
of warning (this is often annoying) or exception (this is
often valuable). Unfortunately, the debugger cannot discern fatal
exceptions from non-fatal ones. If dieLevel
is even 1, then your
non-fatal exceptions are also traced and unceremoniously altered if they
came from eval'ed strings or from any kind of eval within modules
you're attempting to load. If dieLevel
is 2, the debugger doesn't
care where they came from: It usurps your exception handler and prints
out a trace, then modifies all exceptions with its own embellishments.
This may perhaps be useful for some tracing purposes, but tends to hopelessly
destroy any program that takes its exception handling seriously.
AutoTrace
Trace mode (similar to t
command, but can be put into
PERLDB_OPTS
).
LineInfo
File or pipe to print line number info to. If it is a pipe (say,
|visual_perl_db), then a short message is used. This is the
mechanism used to interact with a slave editor or visual debugger,
such as the special vi
or emacs
hooks, or the ddd
graphical
debugger.
inhibit_exit
If 0, allows stepping off the end of the script.
PrintRet
Print return value after r
command if set (default).
ornaments
Affects screen appearance of the command line (see Term::ReadLine). There is currently no way to disable these, which can render some output illegible on some displays, or with some pagers. This is considered a bug.
frame
Affects the printing of messages upon entry and exit from subroutines. If
frame & 2
is false, messages are printed on entry only. (Printing
on exit might be useful if interspersed with other messages.)
If frame & 4
, arguments to functions are printed, plus context
and caller info. If frame & 8
, overloaded stringify
and
tied FETCH
is enabled on the printed arguments. If frame
& 16
, the return value from the subroutine is printed.
The length at which the argument list is truncated is governed by the next option:
maxTraceLen
Length to truncate the argument list when the frame
option's
bit 4 is set.
windowSize
Change the size of code list window (default is 10 lines).
The following options affect what happens with V
, X
, and x
commands:
arrayDepth
, hashDepth
Print only first N elements ('' for all).
dumpDepth
Limit recursion depth to N levels when dumping structures. Negative values are interpreted as infinity. Default: infinity.
compactDump
, veryCompact
Change the style of array and hash output. If compactDump
, short array
may be printed on one line.
globPrint
Whether to print contents of globs.
DumpDBFiles
Dump arrays holding debugged files.
DumpPackages
Dump symbol tables of packages.
DumpReused
Dump contents of "reused" addresses.
quote
, HighBit
, undefPrint
Change the style of string dump. The default value for quote
is auto
; one can enable double-quotish or single-quotish format
by setting it to " or ', respectively. By default, characters
with their high bit set are printed verbatim.
UsageOnly
Rudimentary per-package memory usage dump. Calculates total size of strings found in variables in the package. This does not include lexicals in a module's file scope, or lost in closures.
After the rc file is read, the debugger reads the $ENV{PERLDB_OPTS}
environment variable and parses this as the remainder of a "O ..."
line as one might enter at the debugger prompt. You may place the
initialization options TTY
, noTTY
, ReadLine
, and NonStop
there.
If your rc file contains:
- parse_options("NonStop=1 LineInfo=db.out AutoTrace");
then your script will run without human intervention, putting trace
information into the file db.out. (If you interrupt it, you'd
better reset LineInfo
to /dev/tty if you expect to see anything.)
TTY
The TTY to use for debugging I/O.
noTTY
If set, the debugger goes into NonStop
mode and will not connect to a TTY. If
interrupted (or if control goes to the debugger via explicit setting of
$DB::signal or $DB::single from the Perl script), it connects to a TTY
specified in the TTY
option at startup, or to a tty found at
runtime using the Term::Rendezvous
module of your choice.
This module should implement a method named new
that returns an object
with two methods: IN
and OUT
. These should return filehandles to use
for debugging input and output correspondingly. The new
method should
inspect an argument containing the value of $ENV{PERLDB_NOTTY}
at
startup, or "$ENV{HOME}/.perldbtty$$"
otherwise. This file is not
inspected for proper ownership, so security hazards are theoretically
possible.
ReadLine
If false, readline support in the debugger is disabled in order to debug applications that themselves use ReadLine.
NonStop
If set, the debugger goes into non-interactive mode until interrupted, or programmatically by setting $DB::signal or $DB::single.
Here's an example of using the $ENV{PERLDB_OPTS}
variable:
- $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
That will run the script myprogram without human intervention,
printing out the call tree with entry and exit points. Note that
NonStop=1 frame=2 is equivalent to N f=2
, and that originally,
options could be uniquely abbreviated by the first letter (modulo
the Dump*
options). It is nevertheless recommended that you
always spell them out in full for legibility and future compatibility.
Other examples include
- $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram
which runs script non-interactively, printing info on each entry
into a subroutine and each executed line into the file named listing.
(If you interrupt it, you would better reset LineInfo
to something
"interactive"!)
Other examples include (using standard shell syntax to show environment variable settings):
- $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
- perl -d myprogram )
which may be useful for debugging a program that uses Term::ReadLine
itself. Do not forget to detach your shell from the TTY in the window that
corresponds to /dev/ttyXX, say, by issuing a command like
- $ sleep 1000000
See Debugger Internals in perldebguts for details.
The debugger prompt is something like
- DB<8>
or even
- DB<<17>>
where that number is the command number, and which you'd use to
access with the built-in csh-like history mechanism. For example,
!17
would repeat command number 17. The depth of the angle
brackets indicates the nesting depth of the debugger. You could
get more than one set of brackets, for example, if you'd already
at a breakpoint and then printed the result of a function call that
itself has a breakpoint, or you step into an expression via s/n/t
expression command.
If you want to enter a multi-line command, such as a subroutine definition with several statements or a format, escape the newline that would normally end the debugger command with a backslash. Here's an example:
- DB<1> for (1..4) { \
- cont: print "ok\n"; \
- cont: }
- ok
- ok
- ok
- ok
Note that this business of escaping a newline is specific to interactive commands typed into the debugger.
Here's an example of what a stack backtrace via T
command might
look like:
- $ = main::infested called from file `Ambulation.pm' line 10
- @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
- $ = main::pests('bactrian', 4) called from file `camel_flea' line 4
The left-hand character up there indicates the context in which the
function was called, with $
and @
meaning scalar or list
contexts respectively, and . meaning void context (which is
actually a sort of scalar context). The display above says
that you were in the function main::infested
when you ran the
stack dump, and that it was called in scalar context from line
10 of the file Ambulation.pm, but without any arguments at all,
meaning it was called as &infested
. The next stack frame shows
that the function Ambulation::legs
was called in list context
from the camel_flea file with four arguments. The last stack
frame shows that main::pests
was called in scalar context,
also from camel_flea, but from line 4.
If you execute the T
command from inside an active use
statement, the backtrace will contain both a require frame and
an eval) frame.
This shows the sorts of output the l
command can produce:
- DB<<13>> l
- 101: @i{@i} = ();
- 102:b @isa{@i,$pack} = ()
- 103 if(exists $i{$prevpack} || exists $isa{$pack});
- 104 }
- 105
- 106 next
- 107==> if(exists $isa{$pack});
- 108
- 109:a if ($extra-- > 0) {
- 110: %isa = ($pack,1);
Breakable lines are marked with :
. Lines with breakpoints are
marked by b
and those with actions by a
. The line that's
about to be executed is marked by ==>.
Please be aware that code in debugger listings may not look the same as your original source code. Line directives and external source filters can alter the code before Perl sees it, causing code to move from its original positions or take on entirely different forms.
When the frame
option is set, the debugger would print entered (and
optionally exited) subroutines in different styles. See perldebguts
for incredibly long examples of these.
If you have compile-time executable statements (such as code within
BEGIN and CHECK blocks or use statements), these will not be
stopped by debugger, although requires and INIT blocks will, and
compile-time statements can be traced with AutoTrace
option set
in PERLDB_OPTS
). From your own Perl code, however, you can
transfer control back to the debugger using the following statement,
which is harmless if the debugger is not running:
- $DB::single = 1;
If you set $DB::single
to 2, it's equivalent to having
just typed the n
command, whereas a value of 1 means the s
command. The $DB::trace
variable should be set to 1 to simulate
having typed the t
command.
Another way to debug compile-time code is to start the debugger, set a breakpoint on the load of some module:
- DB<7> b load f:/perllib/lib/Carp.pm
- Will stop on load of `f:/perllib/lib/Carp.pm'.
and then restart the debugger using the R
command (if possible). One can use b
compile subname
for the same purpose.
The debugger probably contains enough configuration hooks that you
won't ever have to modify it yourself. You may change the behaviour
of debugger from within the debugger using its o
command, from
the command line via the PERLDB_OPTS
environment variable, and
from customization files.
You can do some customization by setting up a .perldb file, which contains initialization code. For instance, you could make aliases like these (the last one is one people expect to be there):
- $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
- $DB::alias{'stop'} = 's/^stop (at|in)/b/';
- $DB::alias{'ps'} = 's/^ps\b/p scalar /';
- $DB::alias{'quit'} = 's/^quit(\s*)/exit/';
You can change options from .perldb by using calls like this one;
- parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
The code is executed in the package DB
. Note that .perldb is
processed before processing PERLDB_OPTS
. If .perldb defines the
subroutine afterinit
, that function is called after debugger
initialization ends. .perldb may be contained in the current
directory, or in the home directory. Because this file is sourced
in by Perl and may contain arbitrary commands, for security reasons,
it must be owned by the superuser or the current user, and writable
by no one but its owner.
You can mock TTY input to debugger by adding arbitrary commands to @DB::typeahead. For example, your .perldb file might contain:
- sub afterinit { push @DB::typeahead, "b 4", "b 6"; }
Which would attempt to set breakpoints on lines 4 and 6 immediately after debugger initialization. Note that @DB::typeahead is not a supported interface and is subject to change in future releases.
If you want to modify the debugger, copy perl5db.pl from the
Perl library to another name and hack it to your heart's content.
You'll then want to set your PERL5DB
environment variable to say
something like this:
- BEGIN { require "myperl5db.pl" }
As a last resort, you could also use PERL5DB
to customize the debugger
by directly setting internal variables or calling debugger functions.
Note that any variables and functions that are not documented in this document (or in perldebguts) are considered for internal use only, and as such are subject to change without notice.
As shipped, the only command-line history supplied is a simplistic one that checks for leading exclamation points. However, if you install the Term::ReadKey and Term::ReadLine modules from CPAN, you will have full editing capabilities much like GNU readline(3) provides. Look for these in the modules/by-module/Term directory on CPAN. These do not support normal vi command-line editing, however.
A rudimentary command-line completion is also available. Unfortunately, the names of lexical variables are not available for completion.
If you have the FSF's version of emacs installed on your system, it can interact with the Perl debugger to provide an integrated software development environment reminiscent of its interactions with C debuggers.
Perl comes with a start file for making emacs act like a syntax-directed editor that understands (some of) Perl's syntax. Look in the emacs directory of the Perl source distribution.
A similar setup by Tom Christiansen for interacting with any vendor-shipped vi and the X11 window system is also available. This works similarly to the integrated multiwindow support that emacs provides, where the debugger drives the editor. At the time of this writing, however, that tool's eventual location in the Perl distribution was uncertain.
Users of vi should also look into vim and gvim, the mousey and windy version, for coloring of Perl keywords.
Note that only perl can truly parse Perl, so all such CASE tools fall somewhat short of the mark, especially if you don't program your Perl as a C programmer might.
If you wish to supply an alternative debugger for Perl to run, just invoke your script with a colon and a package argument given to the -d flag. The most popular alternative debuggers for Perl is the Perl profiler. Devel::DProf is now included with the standard Perl distribution. To profile your Perl program in the file mycode.pl, just type:
- $ perl -d:DProf mycode.pl
When the script terminates the profiler will dump the profile information to a file called tmon.out. A tool like dprofpp, also supplied with the standard Perl distribution, can be used to interpret the information in that profile.
use re 'debug'
enables you to see the gory details of how the Perl
regular expression engine works. In order to understand this typically
voluminous output, one must not only have some idea about how regular
expression matching works in general, but also know how Perl's regular
expressions are internally compiled into an automaton. These matters
are explored in some detail in
Debugging regular expressions in perldebguts.
Perl contains internal support for reporting its own memory usage, but this is a fairly advanced concept that requires some understanding of how memory allocation works. See Debugging Perl memory usage in perldebguts for the details.
You did try the -w switch, didn't you?
perldebtut, perldebguts, re, DB, Devel::DProf, dprofpp, Dumpvalue, and perlrun.
When debugging a script that uses #! and is thus normally found in
$PATH, the -S option causes perl to search $PATH for it, so you don't
have to type the path or which $scriptname
.
- $ perl -Sd foo.pl
You cannot get stack frame information or in any fashion debug functions that were not compiled by Perl, such as those from C or C++ extensions.
If you alter your @_ arguments in a subroutine (such as with shift
or pop), the stack backtrace will not show the original values.
The debugger does not currently work in conjunction with the -W command-line switch, because it itself is not free of warnings.
If you're in a slow syscall (like waiting, accepting, or reading
from your keyboard or a socket) and haven't set up your own $SIG{INT}
handler, then you won't be able to CTRL-C your way back to the debugger,
because the debugger's own $SIG{INT}
handler doesn't understand that
it needs to raise an exception to longjmp(3) out of slow syscalls.