Sounds right on

# For each record:
push(@{$logged_in{$fields[2]}}, $fields[0]);

# Sample usage:
foreach my $loc (sort keys %logged_in) {
   foreach my $user (@{$logged_in{$loc}}) {
      print("$user logged in at location $loc.\n");
   }
}
[download]

or if you want the users only listed at most once per location:

# For each record:
$logged_in{$fields[2]}{$fields[0]} = 1;

# Sample usage:
foreach my $loc (sort keys %logged_in) {
   foreach my $user (sort keys %{$logged_in{$loc}}) {
      print("$user logged in at location $loc.\n");
   }
}
[download]