A few style remarks:

1. try 'use strict' for a change.
2. try to avoid reeling in outside info in your namespace... use nested structures instead. perlref, perldsc, perllol to name a few. A link to perlreftut can be found in my homenode.
3. read up on sort and perlop. Use these kind of constructs to sort nested structures

   @sorted_keys = sort { $HoH{$b}{'number'} <=> $HoH{$a}->{'number'} } ke
   +ys %HoH;
   [download]

4. The require 'config.txt' looks questionable, to say the least. Try to use a real config format, like AppConfig.
5. Try to minimalize the use of reserved words (see perlfunc ). They are very confusing..

So, your code:

open(FILE,$log);
flock (FILE,3);
@users=<FILE>;
close(FILE);
foreach $lines (@users){
chop($lines);
($ips,$times,$locationold)=split('×',$lines );
$timeoff=$time-$times;
push @locations,$locationold if (!$exists{$locationold});
$exists{$locationold}=1;
${$locationold}{online}++;
foreach $loc (@locations){
if(${$loc}{online} < "2") {
${$loc}{users} = "user";
} else {
${$loc}{users} = "users";
}
}
foreach $loc (@locations){
print "${$loc}{online} ${$loc}{users} $loc\n";
}
[download]

Could be written as:

use strict;
use warnings;
use CGI::Carp qw/fatalsToBrowser/;

open(FILE,$log) or die "Couldn't open $log: $!";
flock (FILE,3) or die "Couldn't lock $log: $!";
my @users=<FILE>;
close(FILE);
chomp @users;

my %locations;
foreach my $lines (@users){
  my ($ips,$times,$location)=split('×',$lines);
  $timeoff=$time-$times;
  if ( not exists $locations{$location}) {
    $locations{$location}={online => 1};
  } else {
    $locations{$location}->{'online'}++;
  }
}
$location{$_}->{'usertext'}=$location{$_}->{'online'}==1? 'user' : 'us
+ers' for keys %locations;

foreach my $loc (sort { $b->{'online'} <=> $a->{'online'} }keys %locat
+ions){
  my $href = $locations{$loc};
  print $href->{'online'}." ".$href->{'usertext'}."$loc\n";
}
[download]

To start with. I have added: Hashes of hashes, strict compliance (sort of), die statement as open check, fatalstobrowser, and more things.... it's only a start. Code still unchecked, may contain typos etc.

Hope this gets you on your way....

Jeroen
"We are not alone"(FZ)

Updated... still more to come, I guess.