I don't know why it says that; I can't see how it's that useful for interpreting other languages. A couple of paragraphs further down it says:

It's important to know what custom operators won't do for you. They won't let you add new syntax to Perl, directly. They won't even let you add new keywords, directly. In fact, they won't change the way Perl compiles a program at all.

If you're building your own interpreter for another language into Perl, you'd first need to sort out the parsing. The custom op still would only be an optimization stage afterwards to enable the language's keywords to perform comparably to Perl's built-ins.