$base_url special meaning?

punch_card_don has asked for the wisdom of the Perl Monks concerning the following question:

Mordant Monks,

Scratching my head over this. This snippet is in a script accessed over the web:

print "<p>base_url = $base_url\n";
$base_url = 'http://123.456.789.000';
print "<p>base_url = $base_url\n";
[download]

which usually correctly produces

base_url = 
base_url = http://123.456.789.000
[download]

But one user is accessing my script through a proxy server. (EZProxy, what else?) When they access it, they get:

base_url = 
base_url = http://proxy.theirdomain.com:xxxx
[download]

The variable $base_url is hard-coded in the script. The fact of coming from a proxy server should have zero effect on that. Now, this script uses only one module, CGI.

Am I unwittingly referencing a special variable or something that's overridding my hard-coded value?

Thanks

UPDATE

Immediately following the above snippet, there comes:

print "<p>at A called_from = $called_from\n";
print "<p>at A base_url = $base_url\n";
@url_parts = split (/$base_url.*?\// ,$called_from);
print "<p>at B base_url = $base_url\n";
for $i (0 .. $#url_parts) {
    print "<br>url_parts[$i] = $url_parts[$i]\n";
}
[download]

where "called_from" is the url of the referring page that called the script, set earlier in the script. Again, when I access it, it correctly outputs:

at A called_from = http://123.456.789.000/mysubfolder/mypage.htm
at A base_url = http://123.456.789.000
at B base_url = http://123.456.789.000
url_parts[0] = 
url_parts[1] = mysubfolder/mypage.htm
[download]

but when my proxied friend tries it, they get

at A called_from = http://proxy.theirdomain.com:xxxx/mysubfolder/mypag
+e.htm
at A base_url = http://proxy.theirdomain.com:xxxx
at B base_url = http://proxy.theirdomain.com:xxxx
url_parts[0] = 
url_parts[1] =
[download]

So, OK, I don't get why it's substituting their proxy domain for the hard-coded $base_url - but considering the value of $called_from, url_parts1 should have been the same mysubfolder/mypage.htm as I get - no? Makes me think there's really something weird going on with this $base_url variable.

Time flies like an arrow. Fruit flies like a banana.

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