croak on an attempt to run a directory as a script

croak on an attempt to run a directory as a script

How many times have I meant to run "perl -I lib myprog" but instead
run "perl lib myprog" only to exit 0 because that's what perl does
when you try to run a directory as a script (at least on unix)?  Many.

perl should croak when instructed to execute a directory.
[perl #61362] suggests it already does so on Win32.  Now it does it
everywhere.  Tests not yet written.