This uses the same technique to extract three frame sequences into separate arrays:

my $line ='atgcatccctttaat';
my @trips;
my $frame;
for (unpack('a3X2' x (length($line)-2), $line)) {
  push @{$trips[$frame++]}, $_;
  $frame %= 3;
}
use Data::Dumper;
print Dumper \@trips;
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output:

$VAR1 = [
          [
            'atg',
            'cat',
            'ccc',
            'ttt',
            'aat'
          ],
          [
            'tgc',
            'atc',
            'cct',
            'tta'
          ],
          [
            'gca',
            'tcc',
            'ctt',
            'taa'
          ]
        ];
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