Either you don't need $cnt, or you don't need to iterate over the indexes.

for (0..$#array){
    if ($_ % 20 == 19){
        print OUT "$array[$_]\n";
    }
    else{
        print OUT "$array[$_] ";
    }    
}
[download]

for (@array){
    if (++$cnt % 20 == 0){
        print OUT "$_\n";
    }
    else{
        print OUT "$_ ";
    }    
}
[download]

It's probably worth noting the last line has a trailing space and isn't terminated.