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sub proteintrans {
    my ($dna) = @_;
...
        return $protein;
    }
}

sub proteintrans {
    my ($dna) = @_;
...
    # else return protein (rest of $dna)
    return $protein;
}

#!/usr/bin/perl -w
use strict;
...
                ? $genetic_code{$codon}
                : undef;
}