This is not an error, it is a warning. You can:

1. Ignore it.
2. Turn off this warning. Insert no warnings 'numeric'; into the sort code.

   sub by_cpu {
       no warnings 'numeric';
       $passes->{$a}{cpu} <=> $passes->{$b}{cpu};
   }
   for my $tc (sort by_cpu keys %$passes) {
       # ...
   [download]

3. Filter out the non-numeric values:

   my @dashed;
   my @sorted = sort { $passes->{$a}{cpu} <=> $passes->{$b}{cpu} }
                grep { '-' ne $passes->{$_}{cpu}
                       or push @dashed, $_ and 0
                } keys %$passes;
   for my $tc (@dashed, @sorted) {
       # ...
   [download]

Update: Ouch. no cannot go to the sort code. Changed option 2 and added option 3.