for 1 { say "foo".subst: /(foo)/, {$/[0] ~ "o"}; }

#gives
# Use of Nil in string context  in block  at <unknown file>:1
# o
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Bug 118705 talked about a similar looking set of bugs with the `s///` construct. This construct is a variant of, but not identical to, the `subst` function. It looks like a similar fix will have to be applied separately for `subst`. I've just filed a new `subst` specific bug report.

Afaict the bug is omission of an automatically added `-> $/` signature. One workaround is to manually add the missing piece:

for 1 { say "foo".subst: /(foo)/, -> $/ {$/[0] ~ "o"}; }

»ö« ^{. o O ( "the celebrity tell-all of the Perl-6 cult?" )}