While implementing ffmeg support for transcoding I found that with %w i am always getting negative values. Reason is bug in the TranscodingHelper.pm:

      foreach my $v (keys %vars) {
                my $value;
                
                if ($v eq 's') {$value = "$start";}
                elsif ($v eq 'u') {$value = "$end";}
                elsif ($v eq 't') {$value = Slim::Utils::DateTime::fracSecToMinSec($start);}
                elsif ($v eq 'v') {$value = Slim::Utils::DateTime::fracSecToMinSec($end);}
                elsif ($v eq 'w') {$value = $start - $end;}
                


It should be 

elsif ($v eq 'w') {$value = $end - $start;}


After fixing this I was able to utilize to use ffmpeg and implement cue+ape transcoding.