If I change your code to this (changing your User-Agent to the one used by rexswain.com), and using the normal call to set user-agent, viz:

use LWP::Simple;
use LWP::UserAgent;

$browser = LWP::UserAgent->new();

$browser->agent('Mozilla/5.0 (X11; U; OpenBSD i386; en-US; rv:1.8.1.22
+) Gecko/20090626 SeaMonkey/1.1.17 XpcomViewer/0.9');

$response = $browser->get('http://brtweb.phila.gov/brt.apps/Search/Sea
+rchResults.aspx?id=6546003202');
print $response->content;
[download]

I then get the same amount of lines and text as rexswain.com does, I have not verified the content, can you check? Using your User-Agent string returns a 41437-byte response, and the rexswain User-Agent (used above) returns 43314 bytes, which is the same as the rexswain.com form returns. Perhaps sending Mozilla/4.0 instead of 5.0 was triggering some code path on their ASP code you would not see otherwise.