use warnings;
use strict;

my $len = 600;
my $str = '-' x $len;

for (1 .. 10) {
    my $index = rand($len);

    redo if 'X' eq substr $str, $index, 1;
    substr $str, $index, 1, 'X';
}

my $xCount = $str =~ tr/X/X/;

print "$xCount: >$str<\n";

##</code><code>##