It happens to produce the correct sort in this case because of the narrow data set used.

Actually it doesn't produce a correct sort, its very close, but not correct. But it does match your sort exactly :)

In any case, I find having more than one cmp confusing, so I usually just pad zeros where appropriate, like

#~ @sorted = map { $_->[0] }
@sorted = map { sprintf '%-20s %s', @$_ }
sort { $b->[1] cmp $a->[1] }
map {
    my $f = $_;
    no warnings 'uninitialized';
    my( @didots ) = split /[.]/, ( /-([.\d]+)/ )[0];
    my $ret = sprintf( '%01d.%01d.%01d ', @didots);
## %03d%03d%03d should be sufficiently futureproof 
    
    if(my( $digits ) = /ID(\d+)-/  ){
        $ret = "1 $ret".sprintf( '%03d ', $digits ) 
    } else {
        $ret = "0 $ret"; 
    }
    [ $f, $ret ];
} @unsorted;
print Dumper( \@sorted );
__END__
$VAR1 = [
          'ID15-ABC-6.1         1 6.1.0 015 ',
          'ID3-ABC-6.1          1 6.1.0 003 ',
          'ID12-ABC-5.1.5       1 5.1.5 012 ',
          'ID5-ABC-5.1.5        1 5.1.5 005 ',
          'ID12-ABC-5.1         1 5.1.0 012 ',
          'ID9-ABC-5.1          1 5.1.0 009 ',
          'ID5-ABC-5.1          1 5.1.0 005 ',
          'ABC-6.1              0 6.1.0 ',
          'ABC-5.1.5            0 5.1.5 ',
          'ABC-5.1              0 5.1.0 '
        ];
[download]

Usually pad to 3 digits is sufficient, but sometimes I need to go to 20