Yet another one:

my $seq = 'ACTGACTGATGCATGCATGCATGCCGTACGTACGTACGT';

my @groups = unpack "(A3)*", $seq;
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(But before you generalize this method, note that pre-5.10.0, unpack's "A" template would unpack bytes, not characters... which would make a difference when unicode is involved. This is irrelevant in this particular case, though.)