Are you looking for the /e option? (See perlre and perlretut). Here's an example, in action:

use 5.010_001;
use strict;
use warnings;

my $string = "It's hard to teach an old dog new tricks.";

$string =~ s/(dog)/reverse($1)/e;

say $string;
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The output: "It's hard to teach an old god new tricks." What's happening here is the right-hand side is being evaluated as code. The code should return a string which will serve as the replacement string.