http://search.cpan.org/dist/perl-5.14.0/pod/perldelta.pod

...
Non-destructive substitution

The substitution (s///) and transliteration (y///) operators now support an /r option that copies the input variable, carries out the substitution on the copy, and returns the result. The original remains unmodified.

my $old = "cat";
my $new = $old =~ s/cat/dog/r;
# $old is "cat" and $new is "dog"

This is particularly useful with map. See perlop for more examples.