As a status update: The example from the original post still does not work:
$ perl6-m -e 'grammar G { regex TOP { (foo) <.panic: "!$0!"> }; method
panic($m) { die $m } }; G.parse("foo")'
Use of Nil in string context in regex TOP at -e:1
!!
in method panic at -e:1
in regex TOP at -e:1
in block <unit> at -e:1
The first example from the added irclog gives the expected result now (the
same, niecza gave):
$ perl6-m -e 'say 1001110011 ~~ /^ (.+) $0+ @([\~] $0.comb)? $ /'
Use of Nil in string context in regex at -e:1
｢1001110011｣
0 => ｢10011｣
