I came across this thread several years after the original post, and would like to confirm that this rundll32 solution works perfectly. It is also much better than the Win32::Shell solution, because it doesn't require a module install.

Before finding the answer here, I had tried extracting the http handler program from the registry and calling it directly. That seemed fine until I tried to switch my default browser to Internet Explorer. The exe lives in "C:\Program Files\Internet Explorer\iexplore.exe" so exec() cannot be used because of the spaces in the path. system() worked, but the script doesn't return until the Explorer window is closed.

I have now

my $url = $baseurl . join("&", map {"$_=$params{$_}"} keys %params);
my $cmd = qq{rundll32 url.dll,FileProtocolHandler "$url"};

exec $cmd;
[download]

which works reliably.

The next question will be how to achieve the same thing on Mac and Linux to have a multi-platform script, but that will be in another thread.