FWIW the first two evaluations give identical results now: $ perl6 -e 'my $s = "abcdefghtttaaccta"; my @pats = /ttta<[agt]>cct/, /z|ttta<[agt]>cct/; for @pats -> $pat { say $s.comb( /$pat/ ); }' (tttaacct) (tttaacct)
$ perl6 -e 'my $s = "abcdefghtttaaccta"; my @pats = "ttta<[agt]>cct", "z|ttta<[agt]>cct"; for @pats -> $pat { say $s.comb( /<$pat>/ ); }' (tttaacct) (tttaacct)