my %users_by_account;

...

push @{ $users_by_account{$account_name} }, $username;

...

for my $account_name (sort keys %users_by_account) {
   my $users = $users_by_account{$account_name};

   print("Account $account_name\n");

   for my $username (@$users) {
      print("$username\n");
   }

   print("\n");
}