In this particular case, you don't need to work this way. I entered your data and ended up with this URL:

http://vortex.plymouth.edu/cgi-bin/gen_uacalplt-u.cgi?id=KMIA&pl=none&yy=05&mm=08&dd=24&hh=00&pt=parcel&size=640x480

This produces something that looks like a text file in the browser, as well as in source, but contains 2 lines of extra HTML tags, ending in a "PRE" tag, upfront.

Just download this file, for example with LWP, and you're ready to start parsing.

use LWP::Simple;
getstore('http://vortex.plymouth.edu/cgi-bin/gen_uacalplt-u.cgi?id=KMI
+A&pl=none&yy=05&mm=08&dd=24&hh=00&pt=parcel&size=640x480', 'preKatrin
+aData.txt');
[download]