You need to distinguish between the search pattern and the replace string. To store the compiled search regex, use

    $search = qr/pattern/;
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and to use it in a substitution command, just plug it in:

    s/$search/replace/g;
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To store the entire substitution, I'd go with a subref:

my $rep = sub { $_[0] =~ s/a/b/g };

my $string = "abc";
$rep->($string);
print "$string\n";
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