“ if there are two copies of Perl on the same machine, how do I keep the OS (and me) from getting confused when "perl" is typed on the command line -- will I have to use a different shebang line in scripts? ”

If you're using Macports it will likely be: #!/opt/local/bin/perl and if you install modules through the ports library (e.g., $ sudo port install p5-Data-Dumper). However Macports does not have every module available so you will likely still have to navigate to that directory and install modules from there using MCPAN, cpanminus etc.

To confirm the directory you can just use Mac OSX's Spotlight:

1. (hit the Command and Spacebar keys at the same time). Type in 'Perl'
2. Select 'Show all' at the top of the results. A window will appear.
3. Under the window's search field select the 'plus' icon.
4. Two drop down menus will appear in the row below. Change the second one to 'Folder'
5. You will likely now see at least several folders (directories) with the name 'Perl'. Select the folders to see at the bottom of the window their exact location.

Another tip: If you need to navigate to a specific location within a directory on the command line you can drag and drop that folder to the Mac OSX Terminal (The name of Apple's command line program). That will show the path. Place cd (change directory to:) in front of that path name and it takes you there. A nice trick for moving to something deeply nested.

See:
• Macports Guide: File Hierarchy
• Available Ports: Perl