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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.

print join(", ", reverse "world", "Hello"); # Hello, world

print scalar reverse "dlrow ,", "olleH";    # Hello, world

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

$_ = "dlrow ,olleH";
print reverse;                         # No output, list context
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.

my %by_name = reverse %by_address;  # Invert the hash