I'm lead to believe from what I've read that changing the actual Win32 process name would require a kernel-level driver hack, however if all you want is a nicer name in Task Manager, this works for me:

use Win32::Console;
my $CONSOLE = new Win32::Console();
$CONSOLE->Title($0);
[download]

You might want to take the path off the $0 for a neater result however.

Update: Silly me, I was thinking of the "Applications" tab of Task Manager, which the above does affect, but of course you want the "Processes" tab to change for background processes, so I fall back on the first statement that there doesn't appear to be a way to do that. Even Microsoft gives examples of making multiple copies of EXE fles with different names in order to help organize the Processes tab information. Hmm, I guess you could have your script look for its own name in the process list, and if it doesn't exist, make a copy of PERL.EXE with its own name, and then re-run itself. Ugly.