$ perl -le'
use List::Util q/shuffle/;

my $string_length = 600;
my $x_count       = 10;

my $joined_new_string = q/-/ x $string_length;

my @positions = ( shuffle 0 .. $string_length - 1 )[ 0 .. $x_count - 1 ];

substr $joined_new_string, $_, 1, q/X/ for @positions;

print $joined_new_string;
'
-----------------------------X------------------------------------------------------------------------------------------------------------X-------------------------------------------------------------------------X----X-------X-------------------------------------X----------------------------------------------------------X-X------------------------------------------------------------------------------------------------------------------------------------------------------X-----------------------------------------------------------------------------------------------------X----------------------