Hello, you are using a bareword sort function which is described in What to avoid section of Modern Perl. In the docs for sort is explained that a custom function must return less than 0, 0 or more than 0. The function receives the comparison terms in @_ so you do not need $a and $b.

Anyway if you avoid the --human_redable switch of du and let Perl to humanize it, will have less work to do as in the following pipe of commands:

du -b PATH  | sort -n | perl -ane '++$n and $F[0] /= 1024 until $F[0] 
+< 1024; printf "%.2f %s %s",$F[0], ( qw[ bytes KB MB GB ] )[ $n ], $F
+[1];print qq(\n); $n = 0'
[download]

HtH
L*
Update: see also chapter 15.4. Advanced Sorting in Learning Perl