Now fixed (see modified code and new output below) on ActivePerl, thanks to the previous replies. Has this been a common problem in ActivePerl? I'm pretty new to this, and naively thought the rand function would be platform independent.

$seed = rand(100); 

for ($x=10; $x<10000000; $x=$x*10){
    $error_rate = $x;
    print "Error rate = 1 every $x, errors = ";
    for ($y=0;$y<5;$y++){
        errortest();
    }
    print "\n";
}

sub errortest {
    $errors = 0;
    for ($n=0;$n<1000000;$n++){
        $random = int (better_random($error_rate));
        if ($random == 1){$errors++}
    }
    print $errors, " ";
}

sub better_random { 
    $seed = ($seed * 65539) % 2 ** 31;
    $this_random = $_[0] * ($seed / 2 ** 31);
    return $this_random;
}

Ouput:

Error rate = 1 every 10, errors = 99259 100049 99986 99998 99818
Error rate = 1 every 100, errors = 10166 10098 10056 10107 10032
Error rate = 1 every 1000, errors = 967 1013 1009 948 1004
Error rate = 1 every 10000, errors = 96 108 112 116 104
Error rate = 1 every 100000, errors = 16 11 7 8 10
Error rate = 1 every 1000000, errors = 0 3 1 1 0
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