in code block of subst

diff --git a/src/core/Cool-str.pm b/src/core/Cool-str.pm
index a045cd4..e231260 100644
--- a/src/core/Cool-str.pm
+++ b/src/core/Cool-str.pm
@@ -28,8 +28,19 @@ augment class Cool {
         return self if @matches == 1 && !@matches[0];
         my $prev = 0;
         my $result = '';
+        my $caller_pad = Q:PIR{
+            $P0 = getinterp
+            %r  = $P0['lexpad';1]
+        };
         for @matches -> $m {
             $result ~= self.substr($prev, $m.from - $prev);
+            Q:PIR {
+                $P0 = find_lex '$caller_pad'
+                if null $P0 goto match_set_done
+                $P1 = find_lex '$m'
+                $P0['$/'] = $P1
+                match_set_done:
+            } if $replacement ~~ Callable;
             $result ~= ~($replacement ~~ Callable
                             ?? $replacement($m)
                             !! $replacement);


# this is how it fails:

./perl6 -e 'my $x = "foo"; $x.=subst(/./, "F")'
make: Nothing to be done for `all'.
Null PMC access in can()
current instr.: '&infix:<=>' pc 17802 (src/builtins/Junction.pir:198)
called from Sub 'perl6;Cool;subst' pc 421821 (src/gen/core.pir:0)
called from Sub '!dispatch_.=' pc 438 (src/glue/dispatch.pir:117)
called from Sub '_block14' pc 29 (EVAL_1:0)
called from Sub '!YOU_ARE_HERE' pc 383924 (src/gen/core.pir:29685)
called from Sub '!UNIT_START' pc 1400 (src/glue/run.pir:21)
called from Sub 'perl6;HLL;Compiler;_block432' pc -1 ((unknown file):-1)
called from Sub 'perl6;HLL;Compiler;eval' pc 21950 (ext/nqp-rx/src/stage0/HLL-s0.pir:7074)
called from Sub 'perl6;PCT;HLLCompiler;command_line' pc 1504 (compilers/pct/src/PCT/HLLCompiler.pir:801)
called from Sub 'perl6;Perl6;Compiler;main' pc -1 ((unknown file):-1)