You do use flock, which is good, but if the file is opened while another instance is in the for()-loop, the counter will in effect only be increased once. To avoid this race condition, you have to write to the file without closeing it after reading. This can be done using a read+write mode.

sub counter {
    my ($file) = @_;
    local *COUNTER;
    local $/ = undef;
    open           COUNTER, '+<', $file or
    open           COUNTER, '>',  $file or return undef;
    flock          COUNTER, 2;
    seek           COUNTER, 0, 0;
    my $counter = <COUNTER>;
    seek           COUNTER, 0, 0;
    truncate       COUNTER, 0;
    print          COUNTER ++$counter;
    close          COUNTER;
    return $counter;
}
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- Yes, I reinvent wheels.
- Spam: Visit eurotraQ.

For a guess, you're wondering (did you forget to ask a question?) why this won't compile.

use strict;
use Fcntl ':flock';
#792341

my ($mday,$mon,$year) = (localtime(time))[3,4,5];
$year=$year+1900;

if ($mday < 10) {
    $mday = "0$mday"; 
} 
if ($mon < 10) {
    $mon = "0$mon";
} 
my $month = ++$mon;
my $date = "$mday.$month.$year";

my $file = "counter.txt";
my $host = $ENV{'REMOTE_HOST'};
my $ip = $ENV{'REMOTE_ADDR'};

my ($uq,@counter,@counter_data);

open(COUNTER, "$file") || &error("cannot open file for read");
@counter = <COUNTER>;
close(COUNTER);

for(@counter) { 
    chomp;
     @counter_data = split (/\|/, $_);
     if ($counter_data[0] eq $date && $counter_data1 eq $ip && $counte
+r_d +ata2 eq $host) { 
        $uq = 1;
     } 
} 
if (!$uq) { open (COUNTER, ">>$file") || &error("cannot open file for 
+write");
    flock (COUNTER, 2);
     print COUNTER "$date|$ip|$host\n" or die;
     close(COUNTER) or die;
} 

sub error { 
    my $error = $_[0];
    print "$error\n";
    exit;
}
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Some answers, found with perl -c 792341.pl

Global symbol "$counter_data1" requires explicit package name at 79234
+1.pl line 30.
Global symbol "$counter_d" requires explicit package name at 792341.pl
+ line 30.
Bareword "ata2" not allowed while "strict subs" in use at 792341.pl li
+ne 30.
792341.pl had compilation errors.
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Please see Writeup Formatting Tips and Markup in the Monastery.

And it wouldn't hurt to include use warnings; along with strict.

Update: Missed the missing "[...]". Corrected that and the error messages