sub  sorter ($$) {
    my $am = ( $_[0] =~ /$mask/ ) ? $1 : '';
    my $bm = ( $_[1] =~ /$mask/ ) ? $1 : '';
    $am <=> $bm || $am cmp $bm;
}

# or

sub  sorter ($$) {
    my ( $am ) = ( $_[0] =~ /$mask/ );
    my ( $bm ) = ( $_[1] =~ /$mask/ );
    $am <=> $bm || $am cmp $bm;
}

# note: these are not quite equivalent
# after a failed match, $am and $bm will contain:
#   #1) an empty string
#   #2) undef