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Perl 5 version 20.2 documentation
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  • reverse LIST

    In list context, returns a list value consisting of the elements of LIST in the opposite order. In scalar context, concatenates the elements of LIST and returns a string value with all characters in the opposite order.

    1. print join(", ", reverse "world", "Hello"); # Hello, world
    2. print scalar reverse "dlrow ,", "olleH"; # Hello, world

    Used without arguments in scalar context, reverse() reverses $_ .

    1. $_ = "dlrow ,olleH";
    2. print reverse; # No output, list context
    3. print scalar reverse; # Hello, world

    Note that reversing an array to itself (as in @a = reverse @a ) will preserve non-existent elements whenever possible; i.e., for non-magical arrays or for tied arrays with EXISTS and DELETE methods.

    This operator is also handy for inverting a hash, although there are some caveats. If a value is duplicated in the original hash, only one of those can be represented as a key in the inverted hash. Also, this has to unwind one hash and build a whole new one, which may take some time on a large hash, such as from a DBM file.

    1. %by_name = reverse %by_address; # Invert the hash