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.