When it didn't work, you were using

$string = "atccatccctttaat";
$triplets0 = $line=~ /(...)/;
[download]

Regexps return true/false in scalar context. Fixes:

$string = "atccatccctttaat";
($triplets0) = $line=~ /(...)/;
[download]

or

$string = "atccatccctttaat";
@triplets0 = $line=~ /(...)/;
[download]