Sadly, I can't ignore the warnings as when they appear it means the script has abruptly ended itself (before the real end of the cycle).

No. Global destruction happens at the end of your program, not somewhere in the middle. If you fork(), you maybe get that, so don't do that then.

If you post a complete, self-contained example that reproduces the problem but is not longer than 20 lines, that would help me to help you better. I could then try to reproduce what you're seeing and maybe find what causes it easier.

I managed to make a reduced version of that part of my script, sadly I couldn't manage to reduce it under 50 lines.
Would that be fine for you?

UPDATE: Here is the code and some nicknames to test it:

#!/usr/bin/perl

use strict;
use warnings;

use WWW::Mechanize::Firefox;
use WWW::Mechanize;
use Date::Manip;
use DateTime;

open TXT, "<", "nickname.log";
while(! eof (TXT)){    
    my $nick = <TXT>;
    chomp $nick;
            
    open MATCH, "+<", "match.log";
    
    my $j = 0;
    for $j(0..3){
        
        my ($date) = DateTime->now;
        my ($k) = 0;
        while($k < $j){
            $date = $date->subtract( days => 7);
            $k++;
        }
        $date = $date->ymd;
                        
        my $url = "http://www.quakelive.com/#profile/matches/$nick/$da
+te";
        
        my ($firemech) = WWW::Mechanize::Firefox->new();
        $firemech->get($url);
        die "Cannot connect to $url\n" if !$firemech->success();
                
        my ($retries) = 10;
            while ($retries-- and ! $firemech->is_visible( xpath => '/
+/*[@class="areaMapC"]' )) {
                  sleep 1;
            }
            die "Timeout" unless $retries;

        my ($content) = $firemech->content();

        while(($content =~ /class="areaMapC" id="([^<]+)_([^<]+)_([^<]
++)">/gsi)){
            my ($game) = $1;  my ($longid) = $2;  my ($shortid) = $3;
            my ($matchid) = "$longid/$game/$shortid\n";
            print MATCH $matchid;    
        }
        undef $firemech; undef $content;
        $j++;        
    }
    close MATCH;
}
close TXT;
[download]

Nicknames:

peavey
trajan
komap
bluedawg
donpidon
x9rcist
dom_prova_____
newton
burn
tyryl
st1mz
tomo32
zky
hisarrzulu
_luk__47_
sendhu
joopsta
sand5pid3r
trickle
mikenoid
beg1nner
fladder72
4bsan1ty
daitarn
z0id0uf
amoreno
stuntt
askkk
flyttadig
arcsign
glavic
acid_predator
new_0
jala1
crucci
niko95
deputydawg
optixx
ennesit
martianbuddy
b4r
r1dah
gathion
function9
chup4
poocamshot
shnipzr
heppo
b4byb34r
the_hulk
[download]

It seems that you were fighting two problems. One I can't easily fix - it seems that MozRepl::Client does not like getting instantiated twice. You can fix that by only ever using one instance of WWW::Mechanize::Firefox, outside of your loop. The second problem is that Firefox does not fire the proper events when moving to a new URL, if the new URL only differs in the anchor part from the old URL. I've changed your program to simply do two ->get() calls, one to about:blank to flush the current page, and then one again to the new real URL.

I've also removed the use Date::Manip statement, as it wasn't used at all. There was $j used as a counter but incremented in two places. I've removed the explicit increment. The HTML extraction seemed easier when done using the ->selector() method of WWW::Mechanize::Firefox.

Here is the program that works for me:

#!/usr/bin/perl

use strict;
use warnings;

use WWW::Mechanize::Firefox;
use WWW::Mechanize;
use DateTime;

my ($firemech) = WWW::Mechanize::Firefox->new();
my $count;
while($count++ < 2){    
    my $nick = <DATA>;
    chomp $nick;
            
    #my $j = 0;
    for my $j(0..3){
        
        my ($date) = DateTime->now;
        my ($k) = 0;
        while($k < $j){
            $date = $date->subtract( days => 7);
            $k++;
        }
        $date = $date->ymd;
                        
        $firemech->get('about:blank');
        my $url = "http://www.quakelive.com/#profile/matches/$nick/$da
+te";
        $firemech->get($url);
        die "Cannot connect to $url\n" if !$firemech->success();
                
        my ($retries) = 10;
        while ($retries-- and ! $firemech->is_visible( xpath => '//*[@
+class="areaMapC"]', all => 1 )) {
            sleep 1;
        }
        die "Timeout" unless $retries;

        my ($content) = $firemech->content();

        while(($content =~ /class="areaMapC" id="([^<]+)_([^<]+)_([^<]
++)">/gsi)){
            my ($game) = $1;  my ($longid) = $2;  my ($shortid) = $3;
            my ($matchid) = "$longid/$game/$shortid\n";
            print $matchid;    
        };
        my @areaMapC = $firemech->selector('.areaMapC');
        print $_->{id},"\n" for @areaMapC;
        #$j++;        
    }
}
        undef $firemech;

__DATA__
peavey
trajan
komap
bluedawg
donpidon
x9rcist
dom_prova_____
newton
burn
tyryl
st1mz
tomo32
zky
hisarrzulu
_luk__47_
sendhu
joopsta
sand5pid3r
trickle
mikenoid
beg1nner
fladder72
4bsan1ty
daitarn
z0id0uf
amoreno
stuntt
askkk
flyttadig
arcsign
glavic
acid_predator
new_0
jala1
crucci
niko95
deputydawg
optixx
ennesit
martianbuddy
b4r
r1dah
gathion
function9
chup4
poocamshot
shnipzr
heppo
b4byb34r
the_hulk
[download]