Improve post-puppy.pl.

When using chunked encoding:
* $ENV{"CONTENT_LENGTH"} is not set.
* $q->param("POSTDATA") is empty.
* <STDIN> contains the full request body.

When not using chunked encoding:
* $ENV{"CONTENT_LENGTH"} is set.
* $q->param("POSTDATA") contains the full request body.
* <STDIN> has already been read.