You probably want to define something testable. It is more interesting to have some type of competition.

Low level

Understand language data types, control structures, etc. Able to distinguish Perl from other languages and leverage the positive features. Produced enough programs to avoid some of the obvious newbie mistakes and frequently asked questions. Able to write most of the code in a small program without a trial and error approach. Familiar with most modules in the standard distribution. A test might be to write a small program without referring to a book or perldoc for everything. A competition might be to code a simple program the fastest and/or most efficient.

Medium level

Familiar with enough of the available CPAN modules to avoid reinventing the wheel. At this level, a programmer should be a to independently write a non-trivial program using some modules not in the standard distribution. A programmer at this level knows the esoteric features of the language and when to apply them. A test might be a client/server or special purpose application using some degree of originality and innovation. One possible competition is the quick solution of a broken program or obfuscated piece of code.

High level

Able to teach, mentor, train low or high level programmers (as seen on Perl Monks). We don't have any "Larry Wall" level developers at work. :-) Typically the high level developer has informally established expertise with other programmers in at least one domain. A high level developer knows the how and why of developing in Perl and the available resources. A test might be the development of a non-trivial module the vastly simplifies the development of an application or service. Usually a high-level programmer has established themselves and isn't competing with the technical skills of others (egos are another story).