[perl #132120] [REGEX] Accessing captures from another capture fails a match

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IRC: https://irclog.perlgeek.de/perl6/2017-09-18#i_15181575

We can use $0 here to refer to previous capture and regex matches:
    m: say "11" ~~ /(\d)[$0]/
    rakudo-moar 48a84d: OUTPUT: «｢11｣␤ 0 => ｢1｣␤»

But if we capture that match, the match fails:
    m: say "11" ~~ /(\d)([$0])/
    rakudo-moar 48a84d: OUTPUT: «Nil␤»