my %reverse;
push @{$reverse{$CodonMap{$_}}}, $_ for keys %CodonMap;

##</code><code>##

#!/usr/bin/env perl -l

use strict;
use warnings;

my %CodonMap = (GCA=>'A', GCC=>'A', GCG=>'A', GCU=>'A', UVW=>'B', XYZ=>'B');
my %reverse;
push @{$reverse{$CodonMap{$_}}}, $_ for keys %CodonMap;
print "A codons: @{$reverse{A}}";
print "B codons: @{$reverse{B}}";

##</code><code>##

A codons: GCA GCU GCG GCC
B codons: UVW XYZ