# just return a false value and allow the
# calling code to deal with the false return
(defined($coin) && $coin == 25) || return 0;

# 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);

# use a custom error handler which can record
# call stack information, set error variable,
# and return a specific value.
(defined($coin) && $coin == 25) || return errorHandler("You must insert 25 cents");