Yes, this is perfectly normal. A failing match doesn't clobber the existing match variables ($1, $2, etc).

If you want to do what you're doing, it's probably better to use something like

my ($am) = ($_[0] =~ /$mask/);
my ($bm) = ($_[1] =~ /$mask/);
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though I'd be strongly inclined to lose the ($$) prototype and use $a and $b rather than $_[0] and $_[1]. As well as being more readable, it's also more efficient. (OTOH if efficiency really matters, you may as well follow Fletch's suggestion above.)

I'm baffled by your $am <=> $bm || $am cmp $bm. You know that $am and $bm are either empty or two-digit numbers, so what's the point in cmping them?