Subroutine signatures only get enabled if the signatures feature, either directly through use feature 'signatures'; or implicitly through a feature bundle, like use 5.020;. This feature is now considered stable, and is enabled automatically by use v5.36 (or higher). On versions between 5.020 and 5.034, a warning was issued that signatures were experimental.

The documentation for feature 'signatures' doesn't mention any further implicit activation, so maybe Dave has something like use 5.036; in his environment.