WTF is “0 but true”

Perl variable declaration of a string with the value “0 but true”.

I have spent most of my life coding with JavaScript. For me, it is natural to use, and I know by heart most of its quirks and oddities. I understand many of these are quite strange for newcomers and can generate some aversion. However, recently learnt about one of these things in Perl that makes JavaScript oddities look amateurish.

Perl? What is that?