%reverse= reverse(%CodonMap)

##</code><code>##

use Data::Dumper;
%reverse= reverse(%CodonMap)
print Dumper \%reverse;

##</code><code>##

use Data::Dumper;
my %reverse;

while( my( $k,$v )= each %CodonMap) {
    $reverse{ $v }||= [];
    push @{ $reverse{ $v }}, $k;
};
print Dumper \%reverse;