From my point of view switch processing should be the same in both cases.

There are subtle differences between switches given on the command line, and given on the she-bang line. Switches given on the command line are passed to the perl process when the process starts, using the normal OS way of passing arguments using one of the exec calls. They can be processed beforing doing anything else. Not so for switches on the she-bang line - then perl has to already open a stream for instance. This means for instance that you cannot give a -C on the she-bang line, unless you give it on the command line as well. And there are also subtle differences between perl foo and ./foo when it comes to switches (Too late for "-T" option for instance).

As for the -d switch, I don't know enough of the debugger to be of any help there.