G'day sahusonu,

Welcome to the monastery.

Your simplest solution might be this:

my %reverse;
push @{$reverse{$CodonMap{$_}}}, $_ for keys %CodonMap;
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Here's an example with some additional (bogus) data to show this is extensible:

#!/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}}";
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Output:

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