I just learned that this won't work because the back reference only works from the regex operator, and what I'm doing on the hash values down here is being taken literally.

My question is, what techniques can I look into in order to achieve the desired result?

Eventually, I would like to put the hash on an external text file with pattern on the left side, including grouping, a delimeter, and the replacement in the right side, including back references. 

#!/usr/bin/perl

use strict;

my %fill = (
    #'M(ary)'  => 'G\1',
    'M(ary)'  => 'G$1',
);

while (my $line = (<DATA>)) {
    foreach my $key (keys %fill) {
        if ($line =~ /$key/) {
            $line =~ s/$key/$fill{$key}/;
        }
    }
    print "$line";
}

__DATA__
Mary has a little lamb

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In reply to backreference on hash by marcpascual

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