Perl (and I assume Ruby from the comment) requires the running context of the interpreter to make decisions about how to interpret some code constructs. Because you need the running state, it is not parseable.

Parsing as in determining what constructs different lines represent, without running them. Imagine e.g. a documentation generator that needs to figure out what all your methods and parameters are, but not actually execute them (obviously there are many other use cases).

A friend at an old job had a piece of perl code where one of the lines would be a comment or not when you ran it, depending on user input. He would bring it out whenever someone suggested writing something in perl.

If you emulate the way the MRI parses it, sure.

For example:

  # Is this some_method with an empty block, or an empty hash?
  some_method {}

  # Is this a regex or a comment?
  #
  # It depends how many arguments "whatever" takes, and you
  # don't know that until you run the code that defines it.
  #
  # Interpretation is implementation-defined. It just so
  # happens that the MRI determines that the below ISN'T a
  # regex because it has a SPACE character just after the "/".
  whatever  / 25 ; # / ; raise SystemExit.new;