How to check if the first variable passed into a method is a string. Perl

I have no idea how to check for this. My method(if condition in method) should only work (execute) if the first argument passed in is a string. I know how to check other types, but I can't seem to find anything for checking for a string.

for a hash I would do something like;

if(ref eq 'HASH') {...}

If someone could provide a simple example I'm sure I would be able to apply it to what I'm doing. I will put up the code for the method and an explanation for the whole operational details of the method if needed.