You need a way to transform the directory the file is in to the appropriate top-level directory. Use the File::Basename module, then something like:

sub topdir {
  my($nd,$parent)=@_;
  my $d = $nd;

  # Hack; strip off trailing characters that might be directory
  # seperators
  $parent =~ s/\W$//;

  while ($nd ne $parent)
  {
    $d = $nd;
    $nd = File::Basename::dirname($d);
  }

  return $d;
}
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That HACK line is in case the argument is c:\temp\ instead of just c:\temp. It should really be $dir=join("",fileparse($dir));, but that doesn't work as documented on my system.

The code to use this is:

#!/usr/bin/perl -w

use strict;

use File::Find;
use File::Basename;

our $dir = $ARGV[0];

find(\&do_dir, $dir);

my %hash;
sub do_dir {
 my $size = -s ($File::Find::name)
     or return;
 my $d = topdir($File::Find::dir,$dir);
 if (!defined($hash{$d})) { $hash{$d} = 0; }
# warn "size=$size, d=$d\n";
 $hash{$d} += $size;
}

foreach (keys %hash) {
  print "DIR $_ = $hash{$_}\n";
}
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