Thank you sn1987a! This solved the problem.

I will learn to use Data::Dumper & read the debugger tutorial suggested as well.

SUMMARY:

1) A subroutine to open and read a CSV file worked fine in isolation but stopped working when integrated into larger script.

2) The problem stemmed from an earlier subroutine (open_po) which globally modified the input record separator ($/) through: undef $/;

3) The problem code was modified to change the state locally, according to sn1987a's post to: local ($/);

4) This modification solved the problem.

Thank you all for your help!