'a$1$1c' is a template. You need a template module that understands that format. String::Interpolate should do the trick.

use String::Interpolate qw( );

sub expand {
    my ($rx, $from, $to) = @_;
    if ($from =~ $rx) {
        my $t = String::Interpolate->new();
        $t->($to);
        $t->([ map substr($from, $-[$_], $+[$_] - $-[$_]), 1..$#- ]);
        print "$t\n";
    }
}
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It's close to Perl code ('"a$1$1c"') so you could convert it to Perl code (the step you are missing) and execute it (eval $code), but that's a very bad approach.