This doesn't use split, but is the first thing I think of:

my $string = join '', 'a'..'z';
my @fields = $string =~ /.{1,3}/g;
my @fields2 = grep {$a=!$a} @fields;
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Hmm, $string =~ /.{1,3}/g should even be faster than split /(?(?{pos() % 3})(?!))/, $string.  I guess not as fast as unpack, though.