So far I'm less than impressed.

• The docs are awful. Mixing up builtin apis with documentation-only 'add-ons'.
• The interface is eclectic to say the least. Some apis have to be called as class methods, others as subroutines. Neither are documented.

  setup_for() & toke_scan_word() have to be called as class methods, all the rest (that i've tried) as subroutines.

• It kinda works if all your syntax is contained within one line, but gets messy (if not impossible), if it spreads over multiple lines.
• It doesn't handle nested syntax. Eg. switch( $n ){ case 1:{...} case 2:{...} }

  It has a routine to parse nested tokens, but if you replace the switch leaving the cases as is, it doesn't call the case handler.

  If the syntax above is spread over multiple lines, you can replace the header of the switch okay. Then the case handler is called, but there is no way to tidy up the closing brace of the switch.

Source filters are easier to write and understand. Shame. It looked very interesting (to me).