In the first form, often referred to as a "string eval", the return value of EXPR is parsed and executed as if it were a little Perl program. The value of the expression (which is itself determined within scalar context) is first parsed, and if there were no errors, executed as a block within the lexical context of the current Perl program. This means, that in particular, any outer lexical variables are visible to it, and any package variable settings or subroutine and format definitions remain afterwards.
Note that the value is parsed every time the eval executes. If EXPR is omitted, evaluates $_ . This form is typically used to delay parsing and subsequent execution of the text of EXPR until run time.
If the
unicode_eval feature
is enabled (which is the default under a
use 5.16
or higher declaration), EXPR or $_ is
treated as a string of characters, so use utf8 declarations
have no effect, and source filters are forbidden. In the absence of the
unicode_eval feature,
will sometimes be treated as characters and sometimes as bytes,
depending on the internal encoding, and source filters activated within
the eval exhibit the erratic, but historical, behaviour
of affecting some outer file scope that is still compiling. See also
the evalbytes operator, which always treats its
input as a byte stream and works properly with source filters, and the
feature pragma.
Problems can arise if the string expands a scalar containing a floating
point number. That scalar can expand to letters, such as "NaN"
or
"Infinity"
; or, within the scope of a use locale , the
decimal point character may be something other than a dot (such as a
comma). None of these are likely to parse as you are likely expecting.
In the second form, the code within the BLOCK is parsed only once--at the same time the code surrounding the eval itself was parsed--and executed within the context of the current Perl program. This form is typically used to trap exceptions more efficiently than the first (see below), while also providing the benefit of checking the code within BLOCK at compile time.
The final semicolon, if any, may be omitted from the value of EXPR or within the BLOCK.
In both forms, the value returned is the value of the last expression evaluated inside the mini-program; a return statement may be also used, just as with subroutines. The expression providing the return value is evaluated in void, scalar, or list context, depending on the context of the eval itself. See wantarray for more on how the evaluation context can be determined.
If there is a syntax error or runtime error, or a die
statement is executed, eval returns
undef in scalar context or an empty list in list
context, and $@ is set to the error message. (Prior to
5.16, a bug caused undef to be returned in list
context for syntax errors, but not for runtime errors.) If there was no
error, $@ is set to the empty string. A control flow
operator like last or goto can
bypass the setting of $@ . Beware that using
eval neither silences Perl from printing warnings to
STDERR, nor does it stuff the text of warning messages into
$@ . To do either of those, you have to use the
$SIG{__WARN__} facility, or turn off warnings inside
the BLOCK or EXPR using no warnings 'all'
. See
warn, perlvar, and warnings.
Note that, because eval traps otherwise-fatal errors, it is useful for determining whether a particular feature (such as socket or symlink) is implemented. It is also Perl's exception-trapping mechanism, where the die operator is used to raise exceptions.
If you want to trap errors when loading an XS module, some problems with
the binary interface (such as Perl version skew) may be fatal even with
eval unless $ENV{PERL_DL_NONLAZY}
is set. See
perlrun.
If the code to be executed doesn't vary, you may use the eval-BLOCK form to trap run-time errors without incurring the penalty of recompiling each time. The error, if any, is still returned in $@ . Examples:
Using the eval {}
form as an exception trap in libraries does have some
issues. Due to the current arguably broken state of __DIE__
hooks, you
may wish not to trigger any __DIE__
hooks that user code may have installed.
You can use the local $SIG{__DIE__}
construct for this purpose,
as this example shows:
This is especially significant, given that __DIE__
hooks can call
die again, which has the effect of changing their error
messages:
Because this promotes action at a distance, this counterintuitive behavior may be fixed in a future release.
With an eval, you should be especially careful to remember what's being looked at when:
Cases 1 and 2 above behave identically: they run the code contained in
the variable $x. (Although case 2 has misleading double quotes making
the reader wonder what else might be happening (nothing is).) Cases 3
and 4 likewise behave in the same way: they run the code '$x'
, which
does nothing but return the value of $x. (Case 4 is preferred for
purely visual reasons, but it also has the advantage of compiling at
compile-time instead of at run-time.) Case 5 is a place where
normally you would like to use double quotes, except that in this
particular situation, you can just use symbolic references instead, as
in case 6.
Before Perl 5.14, the assignment to $@ occurred before restoration of localized variables, which means that for your code to run on older versions, a temporary is required if you want to mask some but not all errors:
eval BLOCK
does not count as a loop, so the loop control statements
next, last, or
redo cannot be used to leave or restart the block.
An eval ''
executed within a subroutine defined
in the DB
package doesn't see the usual
surrounding lexical scope, but rather the scope of the first non-DB piece
of code that called it. You don't normally need to worry about this unless
you are writing a Perl debugger.