To answer your question: yes, that is normal behaviour. $1 and friends are only set on a
succesful match - if the match fails, $1 and friends keep their value. You could do:
$am = $1 if $_[0] =~ /$mask/;
$bm = $1 if $_[1] =~ /$mask/;
Or, to avoid warnings:
($am, $bm) = map {/$mask/ ? $1 : 0} @_;
Having said that, I always try to avoid using a sort-sub, and often a sort-block as well. Instead, I would opt for a GRT in this case:
@result = map {substr $_, 2}
sort
map {sprint "%02d%s", (/$mask/ ? $1 : 0), $_} @f;
Using a GRT means you do the extraction of the key you want to sort on only once per element, instead of once per comparison. If you have to sort a long array, this can be significant.