use Carp;

##</code><code>##

{
    local $| = 1;
    # code needing unbuffered output here
}

##</code><code>##

$ perl -Mstrict -Mwarnings -le '
    sub codon_to_aa {
        my ($codon) = @_;

        for ($codon) {
            return 
                  /GC./i    ? "A"
                : /TG[TC]/i ? "C"
                : /GA[TC]/i ? "D"
                : do { print "Sorry"; "" };
        }
    }

    for (qw{GCT TGT XXX}) {
        print "$_ : ", codon_to_aa($_);
    }
'
GCT : A
TGT : C
Sorry
XXX :