Interesting. So a failed match will clear the $1 variable. I'm still a little stumped by Marco's original example:

$test = <<'END';
XXWY
XXWZ
END

$count = $test =~ s#(XXW?Y)##gi;
print "REMOVED: <<$1>>\nCOUNT: $count\n";
[download]

That fails. But remove the "i" in the substitution and $1 is defined. Why would case-insensitive make any difference?
--rjk