Wait for one particular element, yes that could be it.
* time passes and i search *
I found it !
I had seen it several weeks ago, but had not understood at the time, and then forgotten.
In the cookbook, HERE.

Here is my version, and it doesn't work. When i call the WWW::Mechanize::Firefox::is_visible() subroutine, it stops in the middle and says "10 elements found for '//a@class="kl"' at test_events_synchronize.pl line 51."
Apparently this happens in find_link_dom(), and it highlights many elements in the window. What am i doing wrong? Here is my code :

#!/usr/bin/perl 

use 5.012003;    # Perl version requirement

use strict;
use warnings;    #comment before releasing

use autodie              2.10;
use Carp                 1.25;
use Perl6::Say           0.16;
use Readonly             1.03;
use Scalar::Util                qw( dualvar );
#
#####

use WWW::Mechanize::Firefox 0.59;
use Time::HiRes             1.9721  qw( time );

# where is my Firefox executable ? 
Readonly my $FIREFOX_FOLDER 
    => 'C:\Program Files\Mozilla Firefox\firefox.exe';
#
######

# URL address of the two pages that we want to open in two tabs
Readonly my $URL => 'https://www.google.ca/search?tbm=isch&hl=en&sourc
+e=hp&biw=1400&bih=866&q=big+images&gbv=2&oq=big+images&aq=f&aqi=g10&a
+ql=&gs_sm=3&gs_upl=1564l3028l0l3402l10l10l0l0l0l0l142l1032l4.6l10l0&g
+s_l=img.3..0l10.1564l3028l0l3403l10l10l0l0l0l0l142l1032l4j6l10l0';

# start time
my $t_start = time;

# start browser 1
my $browser = WWW::Mechanize::Firefox->new(
    # launch Firefox if it is not already running
    launch => $FIREFOX_FOLDER,
    
    # events to wait for, before continuing after get()ing a page
#    events => ['DOMContentLoaded'], 
    synchronize => 0,
);
croak 'Could not initialize the browser' if !$browser->success();
say '# Browser is now ready to go';

# open page
$browser->get( $URL );
croak 'Could not open the page' if !$browser->success();
say '# Page opened';

my $test_to_execute = 1;

if( $test_to_execute == 1 ) { 
    # wait until one particular element appears 
    my $retries = 10;
    while ($retries-- 
           && ! $browser->is_visible( xpath => '//a[@class="kl"]' )) {
        sleep 1;
    };
    die "Timeout" unless $retries;
}
elsif(  $test_to_execute == 2 ) { 
    my $test_is_visible = $browser->is_visible( xpath => '//a[@class="
+kl"]' );
    say $test_is_visible;
}

# print time passed
my $t_passed = dualvar time - $t_start, 'time passed';
say "$t_passed = " . ($t_passed+0);

# stop the program, so i can check which pages have been opened
say '# done !';
my $stop = <ARGV>
[download]

PS : if is set

$test_to_execute = 2
[download]

->is_visible expects your XPath expression to find 0 or 1 element, not more. If you know that there will be zero or multiple elements, you can restrict your XPath expression by using an index:

//a[@class="kl"][1]
[download]

It doesn't work if i use

 xpath => '//a[@class="kl"][1]'
[download]

(i get exactly the same error).
But it works when i use

 xpath => '//input[@class="gbqfif"]'
[download]

which is a unique element in the page.
The result is perfect : 2 seconds or less instead of 5 or 6! Thank you so much!

I hope you're having good times :o)

PS: i forgot to say yesterday that in the cookbook (here) the code is (slighly) broken:
When $retries reaches 0, then it becomes equal to -1 and the "Timeout" exception is not triggered.
It's just a detail, but i thought it might help others if it was fixed : i waited for 10 seconds many times before realizing that is was not the page charging, but just a Timeout.