I assume you're saying that because you're accessing the data again in "scan_matrix". So here's what's really happening.

1. Each time round the loop new lexical variables are created by the "my" call.
2. You then call "get_elements" which puts data into the _package_ variables called @source and @destination, _not_ the lexical variables that you have created.
3. You then call "scan_matrix" which also accesses the _package_ variable versions.
4. You then use "undef" in your main loop which effects the _lexical_ versions of the variables.
5. The loop ends and your lexical variables go out of scope and are destroyed. The package variables, however, still exist and still contain the data that "get_elements" put in them.
6. The loop executes again and the process repeats.

dave, many thanks for this clarification... i obviously made a big mistake! i'll test it now...
--
to ask a question is a moment of shame
to remain ignorant is a lifelong shame

EVERYBODY: thanks for your replies, i changed the my (@source,@destination) into local (@source,@destination) and now it works like it should.

so thanks again for this simple lesson..
--
to ask a question is a moment of shame
to remain ignorant is a lifelong shame