print "$date_a\n$date_b\n";
print "Do these dates differ by more than 72 hours? ";
my $ans = <>;

my $differ_by_more_than_72;
if( $ans =~ /[Yy][Ee][Ss]/ ) {
  $differ_by_more_than_72 = 1;
} else {
  $differ_by_more_than_72 = 0;
}
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Of course if you'd given more information like what format your input dates are in someone might could give you a slightly better answer, but other than the above you're probably just going to get pointers at some Date module on CPAN.