I'm ashamed to admit that I've often wondered the same things. It's not specific to Perl though, it's rather a question about security in Unix.

I very much doubt it is possible to access the memory space of a program running under an other UID. Otherwise that would be an obvious security vulnerability. Same for standard input and output. Can you imagine if you could see the standard output of any terminal running on your machine, including other users's ones? That'd make Unix a joke in terms of security.