Relevant:
https://rt.perl.org/Ticket/Display.html?id=123934#txn-1401917
In short, `||` alternations don't respect `:` in Rakudo, whereas `|`
alternations (and other atoms such as quantifiers) do respect it.
Simpler test-case:
➜ say "ab" ~~ / [ "ab" | "a" ]: "b" /;
Nil
➜ say "ab"
# New Ticket Created by Aleks-Daniel Jakimenko-Aleksejev
# Please include the string: [perl #130117]
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# https://rt.perl.org/Ticket/Display.html?id=130117 >
*Code:*
say ‘abcz’ ~~ /:r [‘a’ || ‘abc’ ] ‘z’ /
*Result:*
｢abcz｣
:r
