do you mean: anyone interested would have to download the packed script ?
Everything could still be packed into one distribution: the local webserver script plus the files it serves to the local browser, which would include the WebPerl code.
Why using WebPerl to build the GUI ?
It's unfortunately also currently not possible to just stick any interactive command-line script into WebPerl, because reading from STDIN is blocking, and doing blocking I/O, while not impossible, requires some pretty complex workarounds. (I've got this in the back of my head, and if I see some possible workarounds showing up, I'd see if I could implement those.) So while some Perl code can be copied directly into WebPerl, other Perl code requires one to think about the limitations inherent in the browser environment.