With reference to your update, you were pretty close, you just needed to actually compare the values to went to the trouble of extracting :) ...

sub files_sort {
    my $first_number; 
    my $second_number; 
    if ($a =~ /(\d+)_(\d+)_duration_(\w+)_comptage_(\d+).txt$/){
        $first_number = $1; 
    }
    if ($b =~ /(\d+)_(\d+)_duration_(\w+)_comptage_(\d+).txt$/){
        $second_number = $1; 
    }

    $first_number <=> $second_number;
}
[download]

Have a look in 'perldoc perlop' if you don't know what the 'space ship' operator does, it's _very_ handy for sort functions, and is doing what you were trying to do with your if's and elses.

I would add though, that if this was my code, I'd probably write a sub to extract the index number from the file name, and write the whole thing like this.

sub my_sort {
    parse_file_name($a) <=> parse_file_name($b);
}
sub parse_file_name {
    if ($_[0] =~ /(\d+)_\d+_duration_\w+_comptage_\d+.txt$/) {
        return $1;
    } else {
        return 0;
    }
}
[download]