Minor nitpick:

(defined($coin) && $coin == 25) || warn("You must insert 25 cents"), return 0;

Should be either

(defined($coin) && $coin == 25) or warn("You must insert 25 cents"), return 0;

or

(defined($coin) && $coin == 25) || (warn("You must insert 25 cents"), return 0);

Otherwise it will always return. I personally always use or for these kinds of idioms, as it is more likely to have the precedence I want.