One way:

c:\@Work\Perl\monks>perl -wMstrict -le
"my %fill = (
   '\bM(ary)\b'  => 'G$1',
   '\bL(arry)\b' => 'B$1',
   );
 ;;
 my $s = 'Mary has a little lamb, Larry has a big one';
 print qq{'$s'};
 ;;
 foreach my $key (keys %fill) {
   $s =~ s/$key/ qq{qq{$fill{$key}}} /gee;
   }
 print qq{'$s'};
"
'Mary has a little lamb, Larry has a big one'
'Gary has a little lamb, Barry has a big one'
[download]

See also the discussions linked from the Re: Evaluating $1 construct in literal replacement expression node and also its root.