[exim] exim -bem usage

I'm trying to test a variable expansion which is suppose to pull the
value for 'Probability=<some number>%' that is located in a header
called X-PerlMX-Spam.

I have a test email message located in a file called 'test'. I try
calling the following:

--
exim -bem /homes/dhj/test '${extract{Probability}{$h_X-PerlMx-Spam:}}'
--

in the hopes that it will spit out the value of the Probability key.
Unfortunately all its doing is spitting out the file path that I fed
it. That is, its spits out '/homes/dhj/test'.

Any suggestions as to what's wrong here?

Darren....