argument passing as in passing arguments to subroutines?

Absolutely. Make sure that the students understand that they pass a list and can optionally return a list. Explain about the flattening effect of &foo(@a,@b); and explain that they are about to see a way of passing the arrays separately. If you have any advanced students, explain about wantarray.

Is pass by reference the same thing as passing a reference to a subroutine?

I take it you mean (passing a reference) to a subroutine, not passing a (reference to a subroutine). If so, yes, indeed. This is the same as in the C language - except that in C you are dealing with pointers (real addresses), whereas perl has references which essentially do the same job.

Pass by reference: In the calling code, you have a real scalar (or array or hash) e.g. @foo. You pass a reference to it

&mumble(\@foo);
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Now the code inside mumble can modify elements inside @foo:

sub mumble {
    my ($bar) = @_;

    ...
    $bar->[1]++;   #increments second member of array
}
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But what does Data::Dumper do?

Data::Dumper is a useful module that reverse engineers perl data structures into human readable perl code. To reconstruct the data structures, this can be done with an eval - this is one of many ways to do persistence in Perl.

Further reading: perldoc perlsub gives more on subroutines and parameter passing. The Camel book: "Programming Perl" is an invaluable reference. Also, Sriram Srivanam's "Advanced Perl Programming" has an excellent chapter on references.