So, what's in @dna then? Letters? Strings? If you have the sequence in a sequence $dna, you could do:

    print "$_ " for $dna =~ /(?=(..))/g;
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If you have individual letters in an array, you could use:

    print @dna [$_ - 1, $_], " " for 1 .. $#dna;
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Abigail