in reply to
Question regarding variable scope
When you declare my $black_player at the start of the sub .That variable is global through the scope of the sub .
(My or our would behave the same with respect to this sub,behaviour would vary with respect to code followinng down the line).
When you do a
foreach $black_player (@possible_opponents)
This effectively overrides the above $black_player .It is local to the loop.Same principle as a pass by value in C.
Therefore ,you can rename your looping variable or just do what roger has done ,use $_.