Here's my version; I made some changes that I thought cleaned things up a little:

1. This version uses readdir to get the list of home directories, so you can avoid creating a temporary file with ls first;
2. The output goes to STDOUT only. I think that, since your on a *nix machine, you'll find it advantageous not to send your output to a hard-wired data file. If you want your output to go to the screen and a file simultaneously, use the tee command; eg.

   perl my_script.pl | tee my_logfile.dat
   [download]

3. I know this code rather contradicts the quote in my sig, but I couldn't help it. It was too much fun rebuilding the processing into one line. Sorry.

#! /usr/bin/perl

use strict ;
use warnings ;
$|++ ;

use Data::Dumper ;

my $home_root  = '/usr/home' ;
my $size_limit = 30 * 1_024_000 ;

opendir HOME, $home_root or die "Couldn't open $home_root: $!" ;

printf "%-25s | %3.2fMB\n", $_->[0], $_->[1] / 1_024_000
   for grep { $_->[1] > $size_limit }
       map  { [ $_ => ( stat "$home_root/$_/INBOX" )[7] || 0 ] }
       grep { -d "$home_root/$_" } readdir HOME ;

closedir HOME ;

exit ;

__END__
[download]

_______________
DamnDirtyApe

Those who know that they are profound strive for clarity. Those who
would like to seem profound to the crowd strive for obscurity.
            --Friedrich Nietzsche