Anyway, this code works ...

An expression like $positions_ref->[ rand( $#{$positions_ref} ) ] will never evaluate to the last element of the array. Use $positions_ref->[ rand( @{$positions_ref} ) ] instead (unless you want to avoid the last element!).

>perl -wMstrict -le
"my $n = shift || 1000;
 my $arrayref = [ qw{ a b c d e f g i j k l m n o p } ];
 my $last_element = $arrayref->[-1];
 my $n_max_i = 0;
 for (1 .. $n) {
   my $rand_element = $arrayref->[ rand $#$arrayref ];
   ++$n_max_i if $rand_element eq $last_element;
   }
 printf 'last element picked randomly %f%% of %d times',
        $n_max_i/$n*100, $n;
"
last element picked randomly 0.000000% of 1000 times

>perl -wMstrict -le
"my $n = shift || 1000;
 my $arrayref = [ qw{ a b c d e f g i j k l m n o p } ];
 my $last_element = $arrayref->[-1];
 my $n_max_i = 0;
 for (1 .. $n) {
   my $rand_element = $arrayref->[ rand @$arrayref ];
   ++$n_max_i if $rand_element eq $last_element;
   }
 printf 'last element picked randomly %f%% of %d times',
        $n_max_i/$n*100, $n;
"
last element picked randomly 6.700000% of 1000 times
[download]

Updates:

1. Changed examples to use array references.
2. Changed printf statements to correctly calculate percentage.