If all else fails, caching problems can sometimes be worked around by appending some unique dummy parameter to the URL (e.g. use current time in secs, or some counter):

my $url='http://mysite.com/test.html?whatever=1226323589';
[download]

With a bit of luck, this should trick the proxy into believing you're requesting a dynamic resource (which it normally wouldn't cache), or - if that doesn't suffice - the different parameter value would pretend you're requesting a different resource every time...