in reply to How do I create a simple, perl-based text filter for Vim
Since I don't know how to use Vim, this is a guess.
If the _IT_WORKED_ tags are showing up, then the Perl script is running. Perhaps Vim is passing perl the text as a tempfile instead of on standard input. Change <STDIN> to <> and see if that helps.
In Section Seekers of Perl Wisdom