From @softmoth

Rakudo fails to set $0 correctly when using the "$var .= subst()"
self-assignment form​:

[13​:42​:34] $ perl6 -e 'my $r = "VIIII"; my ($m, $d, $c) = <X V I>; $r .=
subst(rx/ ($d)? $c ** 4 /, { $0 ?? "$c$m" !!
"$c$d" }, :g); $r.say;'
IV

But it works correctly with the longer "$var = $var.subst()" form​:

[13​:43​:39] $ perl6 -e 'my $r = "VIIII"; my ($m, $d, $c) = <X V I>; $r =
$r.subst(rx/ ($d)? $c ** 4 /, { $0 ?? "$c$m"
!! "$c$d" }, :g); $r.say;'
IX

[13​:44​:00] $ perl6 --version
This is perl6 version 2013.07-113-g45e8c45 built on parrot 5.5.0 revision
RELEASE_5_5_0

From @FROGGS

Patch​: https://rt-archive.perl.org/perl6//Public/Bug/Display.html?id=119201

Test​: Raku/roast@b023bb0070

The RT System itself - Status changed from 'new' to 'open'

@FROGGS - Status changed from 'open' to 'resolved'