Classes aren't a Perl concept, they're common to all kinds of languages.

And the question wasn't about the perl API, it was a perl API.

I suspect that what the OP wants to hear is that an API is the interface that a vendor documents and provides so that you can write programs to interact with their software. It could be all kinds of different interfaces, such as SOAP or REST, or a bunch of scripts that you run, or a set of libraries that you can use.

Of those, probably the most common is providing a set of libraries with a well-documented interface. If the vendor specifically provides a perl API, then those libraries will almost certainly be in the form of perl modules that you can use in your code, and they might provide an object-oriented (ie, using classes) interface. But the perl interface might instead be procedural, especially if the perl modules provided are just a thin wrapper around C libraries.