One thing to understand is that Perl references and C pointers are two different things (despite the conceptual similarities).  So,

1. Can I pass a reference to a string and have the C-library fill it for me so that I can de-reference it on return and get the string?

The more natural way would to be to create a new scalar value (SV) and place the C string into it, for example using newSVpv(). Then return that SV to the Perl side, where you can access it as a normal Perl string. See perlapi, perlguts and PerlGuts Illustrated.

2. Can I construct and pass a pointer to a C-struct from my perl module and have it return the struct with values filled in?

Perl doesn't have structs in the sense C has. So you have to convert the contents into an appropriate form, such as a hash with keys corresponding to the struct's fields.

3. Can I pass a pointer to a function and have the C library call it

No. See perlcall for how to do it (it comes with several examples).

See also perlxs, perlxstut and Inline::C for the big picture of how to extend Perl with C.

In reply to Re: Extending with C functions - some basic questions by Anonyrnous Monk
in thread Extending with C functions - some basic questions by unlinker

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