I was having a similar problem. You might want to look into $ua->agent("string"); in the LWP docs. It allows you to pass yourself off as whatever browser you wish.
I won't even get into what I think of people who require certain browsers, but unfortunately many do. If they're using server side detection badly, it can thrash the page or completely deny you entry. The answer is to mimic a browser that is allowed.
The most common restriction is a 4+ version browser, followed by requiring IE only. The following string should solve the problem in most cases:
Update: Fixed my horrid horrid late night grammar to just really bad late night grammar. ;)
Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0)