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;
}
[download]

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.

Check Date::Manip or Date::Calc, but don't expect they will solve your homework.

#!/usr/bin/perl -w

use strict;

print CompareDates(
    {
        DATE_1 => "01/10/2006 4:39:11 PM",
        DATE_2 => "02/10/2006 4:39:11 PM",
        LIMIT => 72, # Hours
        PATTERN => '%d/%m/%Y %l:%M:%S %P' # Optional
    }
);

print CompareDates(
    {
        DATE_1 => "01/10/2006 4:39:11 PM",
        DATE_2 => "04/10/2006 4:39:11 PM",
        LIMIT => 72, # Hours
    }
);

print CompareDates(
    {
        DATE_1 => "01/10/2006 4:39:11 PM",
        DATE_2 => "06/10/2006 4:39:11 PM",
        LIMIT => 72, # Hours
    }
);


sub CompareDates {

    my $params = shift;

    use DateTime::Format::Strptime;
    my $parser = DateTime::Format::Strptime->new(
        pattern => $params->{PATTERN} || '%d/%m/%Y %l:%M:%S %P'
    );

    my $dt1 = $parser->parse_datetime($params->{DATE_1});
    my $dt2 = $parser->parse_datetime($params->{DATE_2});
    my $dtchk = $dt2->subtract(hours=>$params->{LIMIT});

    my $pref = "The difference between '$params->{DATE_1}' AND
    '$params->{DATE_2}' is";
    my $suf = "$params->{LIMIT} hours.\n";
    
    return "$pref bigger than $suf" if ($dtchk > $dt1);
    return "$pref equal to $suf" if ($dtchk == $dt1);
    return "$pref smaller than $suf" if ($dtchk < $dt1);

}
[download]

The difference between '01/10/2006 4:39:11 PM' AND
        '02/10/2006 4:39:11 PM' is smaller than 72 hours.
The difference between '01/10/2006 4:39:11 PM' AND
        '04/10/2006 4:39:11 PM' is equal to 72 hours.
The difference between '01/10/2006 4:39:11 PM' AND
        '06/10/2006 4:39:11 PM' is bigger than 72 hours.
[download]