$ perl -E '
$seq = q{NNACGTNNNACGTNACGTNN};
say for split m{[ACGT]N+[ACGT]}, $seq;
say q{-} x 10;
say for $seq =~ m{[ACGT]+}g;'
NNACG
CG
CGTNN
----------
ACGT
ACGT
ACGT
$