Please make sure that you cut and paste the code here, instead of re-writing or writing from scratch in the question box. That would eliminate typographical errors.

my @dna = qw(A T C G);
my $counter;
foreach my $i  (1..$#dna){
    my $base = $dna[$i-1];
    my $pair = $dna[$i];
    print "$base$pair\n";
}
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