On Linux, I use a .forward file of the form:
My wmail.pl file has a line:
and then I process the contents in various ways (e.g.,
filtering some messages, putting some in a file I can read
in a web browser, etc.) Pretty rudimentary, but it works well for me.
(Update-remark: The .forward file sends my emails to the
usual spool via the first line, and then I can do anything
else with it with the script. The first line could be omitted; I included it to have a backup file.)