#!/usr/bin/perl
use warnings;
use strict;

my %accounts = (
  tom =>  "BigApple",
  tom2 => "BigApple2",
  tom3 => "BigApple3",
  tom4 => "BigApple4",
  tom5 => "BigApple5",
  tom6 => "BigApple6",
  tom7 => "BigApple7",
);
# sort the users in descending order
my $counter = 1;
for my $userID (reverse sort keys %accounts) {
  print "$userID\t";
  print "\n" if $counter++ % 3 == 0;
}

__END__
Prints:

tom7	tom6	tom5	
tom4	tom3	tom2	
tom