Thanks for that I realize the line isn't really necessary, but its habit.
Most or all my perl scripts end up being run on Linux.
I went into the file manager > file type associations and changed the .pl to Perl, so I can just type 'hi.pl' and it runs ok.

I think what happened was when ActivePerl got deleted before reinstalation, or my reinstall of XP, the .pl got associated with Command Interpreter cos Perl didn't exist - sounds plausible. The association was not made again when Perl was re-installed - until I manually fixed it.