OK, here's the full code that creates the image password:

$cities = "cities.txt";       # number/letters - city correspondences
$cities_st = "cities_st.txt"; # city standard file
$tmp = "temp.txt";            # temp file to keep the password

# choose random letters or numbers

@array = (0..9,a..z,A..Z);
srand;
foreach (1..5) {    
    $rand = int(rand scalar(@array));    
    push (@selected, $rand);
}

# save the chosen password to file

open TMP, ">$tmp";
print TMP @selected;
close TMP;

# open file with number/letter - cities correspondences

open CITIES, $cities;
while (<CITIES>) {
    for ($x; $x<5; $x++) {
        $match = $selected[$x];
        if (/\+$match\+(\w+)/) {$selected[$x] = $1;}
    }
}
close CITIES;

# erase file with number/letter - city correspondences

unlink $cities;

# read "cities_st.txt"

open CITIES_ST, $cities_st;
while (<CITIES_ST>) {
    /(\w+)/;
    push (@cities, $1);
}
close CITIES_ST;

# randomize cities

foreach (0..99) {
    $rand = int(rand scalar(@cities));
    splice (@cities, $rand, 1);
    push (@cities, $_);
}

# create new "cities.txt"

open CITIES, ">$cities";
foreach (a..z) {
    $city = 0;
    $out = "+",$_,"+",$cities[$city];
    $city++;
    print CITIES $out;
}
foreach (A..Z) {
    $city = 26;
    $out = "+",$_,"+",$cities[$city];
    $city++;
    print CITIES $out;
}
foreach (0..9) {
    $city = 51;
    $out = "+",$_,"+",$cities[$city];
    $city++;
    print CITIES $out;
}
close CITIES;

# print the html code

for ($i; $i<5; $i++){
    print "<img src='image_dir/$selected[$i].jpg' border=0>";
}
[download]

No hidden field is involved, which, of course, would have compromised any kind of security. The script that is called to check the password will read it from the $tmp file, erase it, and erase/create all the image files based on the file ($cities) that contains the new correspondences.

I don't think there is a way that a bot or even a mischievous individual could bypass this password check without hacking into the system first.

PS: Of course, the code that changes the correspondences is only included here for reference purposes. It should be included in the second perl script that checks the password as $cities should change together with the image files.