pp is an export of Data::Dump. Much like Dumper from Data::Dumper, but with better formatting.

The double semicolons are simply a device I use in my REPL to indicate that the input should be evaluated now. It allows me to enter multiple statements and multi-line snippets and have them executed as a single snippet:

[0] Perl>
for my $i ( 1 .. 10 ) {
    print $i;
    if( ( $i % 2 ) == 0 ) {
        print "$i is even";
    }
};;
1
2
2 is even
3
4
4 is even
5
6
6 is even
7
8
8 is even
9
10
10 is even
[0] Perl>
[download]

Thereby allowing me to try out more extensive pieces of code without having to stick it all on one line.

I must have drawn the longer straw, but I'm still curious. I even glanced over the latest perldocs to see if maybe I missed something recent. Perhaps I still have, but didn't find what I was looking for on first run-through of the docs.