Are prior regex captures supposed to be scrubbed once subst method is called?

gistfile1.txt

> my $s = 'aaa123bbb'
aaa123bbb
> $s ~~ m/ ('a' **3) \d**3 ('b' **3)/
｢aaa123bbb｣
 0 => ｢aaa｣
 1 => ｢bbb｣
> my $regex = / ('a' **2) 'a' \d**3 'b' ('b' **2) /
/ ('a' **2) 'a' \d**3 'b' ('b' **2) /
> $s .= subst($regex, $0 ~ "x999y" ~ $1)
aaax999ybbb

I would have expected the result to honour the captures in $regex being evaluated in the subst method call, producing:
aax999ybb

Answer: (By MasterDuke)
$s .= subst($regex, {$0 ~ "x999y" ~ $1})  # need to put the replacement into a block (#callable)