Nice :) It is readable indeed, but it could even be better. This is just a small tip, I do not want to offend you in any way, but I think

# warn the user of the wrong input and return
# false, leaving the calling routine dealing 
# with things
(defined($coin) && $coin == 25) || (warn("You must insert 25 cents"), 
+return 0);
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is better written as

# warn the user of the wrong input and return
# false, leaving the calling routine dealing 
# with things
(defined($coin) && $coin == 25) || return do {warn("You must insert 25
+ cents"); 0};
[download]

IMHO, that would even be more readable