# 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);

##</code><code>##

# 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};