Try this out...

#!/usr/bin/perl

use strict;

my $test = 'ACTGACTGATGCATGCATGCATGCCGTACGTACGTACGT';

for (my $i = 0; $i <= length($test); $i++) {
    print substr($test, $i, 3) . "\n" if ( ($i % 3) == 0 );
}
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