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 ?

You don't say if you've already built a GUI, and if you have, what toolkit you used? WebPerl allows one to basically replace JavaScript, so it's possible to write a fully interactive GUI in the browser, using Perl instead of JS. If you've already got a GUI written with a different toolkit, then it's currently not possible to just put that into WebPerl and have it work.

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.