if ((!($username eq 'sugar'))  && (!($password eq 'spice')))

print "you have entered a wrong password!";

}
[download]

Your logic here is confused (and hard to read), you will get the error message only if the username and password are both wrong, if either one is right the script will continue.

# I would write that check like this:
unless (($username eq 'sugar') && ($password eq 'spice'))

# or, if you don't like unless
if(($username ne 'sugar') || ($password ne 'spice'))
[download]

Also, assuming the assignments to $username and $password near the bottom of your script are where you intend to add this code, you need to rethink your design, it isn't going to work the way you want it to, as the form printing and submission checking are going to be two distinct stages in CGI.