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/;
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Or, to avoid warnings:

    ($am, $bm) = map {/$mask/ ? $1 : 0} @_;
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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;
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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.