Booleans

Rust has a Boolean type (bool) and things like if and while expect things of type bool only. Contrast this to Perl where things like if and while place whatever they're given in boolean context.

Additionally, everything in Rust must be initialized. There are no undefined values.

So in Perl we have a somewhat complicated set of rules about the truthiness and definedness of various expressions and values. In Rust, true is true, false is false, and anything else is going to have to get converted into a Boolean.