If you really want arbitrary positions for every time you run the code, I suggest the following:

my $number_of_positions = 10;
my @positions = (1..200);
for my $i (1..$number_of_positions) {
    $arry[splice(@positions, rand(@positions), 1)] = 15;
}
[download]

Or with a map

my $number_of_positions = 10;
my @positions = (1..200);
map {$arry[splice(@positions, rand(@positions, 1)] = 15;} (1..$number_
+of_positions);
[download]

You just have to tell how many positions you want to be changed.