Another approach. I recommend leaving off the experimental dereferencing stuff but to each her own and it’s your cup of hemlock. :P

#!/usr/bin/env perl
use 5.01;
use strict;
use warnings;

my $dna = shift || "TTATATAAAAGU";
my $matcher = make_matcher();
my @protein;
while ( $dna =~ /\G(?<codon>...)/g ) # Or maybe [ACTGU]{3} + /i
{
    push @protein, $matcher->($+{codon});
}

say $_ || "DERP!" for @protein;

exit;

sub make_matcher {
    my %map =
        ( qr/GC./ => "A",
          qr/TG[TC]/ => "C",
          qr/GA[TC]/ => "D",
          qr/GA[AG]/ => "E",
          qr/TT[TC]/ => "F",
          qr/GG./ => "G",
          qr/CA[TC]/ => "H",
          qr/AT[TCA]/ => "I",
          qr/AA[AG]/ => "K",
          qr/TT[AG]|CT./ => "L",
          qr/ATG/ => "M",
          qr/AA[TC]/ => "N",
          qr/CC./ => "P",
          qr/CA[AG]/ => "Q",
          qr/CG.|AG[AG]/ => "R",
          qr/TC.|AG[TC]/ => "S",
          qr/AC./ => "T",
          qr/GT./ => "V",
          qr/TGG/ => "W",
          qr/TA[TC]/ => "Y",
          qr/TA[AG]|TGA/ => "_" );
    sub {
        my $codon = uc +shift;
        for my $key ( keys %map )
        {
            return $map{$key} if $codon =~ $key;
        }
    };
}
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