use strict;
use warnings;
my $s = "foobar";
my $search=qr/(foo)(bar)/;
my $replace="\$1-\$2";
my $perl = "\$s =~ s/$search/$replace/";
warn $perl;
eval $perl;
print $s;
__OUTPUT__
$s =~ s/(?-xism:(foo)(bar))/$1-$2/ at -e line 1.
foo-bar