LWP::UserAgent has a max_size method that can limit the size that will be retrieved to a fixed number of bytes.

Or you can be more ambitious and pass a :content_cb callback to the get method. This will give you the response in chunks and you can then accumulate them in a variable. When your analysis says that you have enough you can die to end the request, and catch that at a higher level with an eval. This will be a lot more complex, but avoids having to guess how much of the file to download.