I think this has to do with optimization of the regexp. I may not be using terminology quite right, but Perl sees that the $sub variable can be interpolated, so the regexp is eval'd at compile time, thereby populating ${^MATCH}. That is, it is only after being eval'd once that ${^MATCH} gets the value from the search.

The following code also forces a similar 'second eval'.

my $x = "abcde";
$x =~ s/b../sprintf "==${^MATCH}=="/pe; 
print "$x\n";
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