If you are running on Win32, try it this way:

my $pid = open CMD, '|-', qq[\"/full/path/to/iceweasel.exe\" -new-wind
+ow '$url'] or die $!;
close CMD;
...
kill 9, $pid; ## Any numeric value other than 2, 15 or 21 should work 
+here.
[download]

That should start the command directly bypassing cmd.exe and return the pid of the program rather than a shell. It works with opera.exe, but I don't have iceweasel.

Things to note:

1. The first part of the third argument to open should be the fully qualified path and name of the command to run.
2. If the path contains spaces, then you must delimit it using escaped double quotes as shown.
3. If any of the arguments contain spaces, you may have to delimit them in the same way.
4. The usual *nix signals 'INT', 'TERM' & 'BREAK' don't work on Win32, but *any* other signal except 0, including those that would not be fatal under *nix, will be.

Which OS are you on?