The weird part here is that $tsize[$tnum] has pretty much no useful meaning in the code. In a list context, @{$threads{$thread}} has all the values in $threads{$threads}. In a scalar context @{$threads{$thread}} is the number of things in the @{$threads{$thread}} list.

$tsize[$tnum] = @{$threads{$thread}};
doesn't appear to have any real meaning or usefulness.

use @{$threads{$thread}} instead of trying to assign a scalar value to a 'C' style @tsize array.