From perlvar, $$ is the process ID of the perl running your script.

With regards to \@ and \$: I'm assuming that you're talking about something like:

    my $array_ref = \@array;
    my $hash_ref  = \%hash;
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Is that the case?

What's happening there is that you're taking a reference to an array and a hash. References are described in perlref. They're kind of like C pointers, but also kind of not. :)

References are scalars that "point" to other variables--the other variables can be arrays, hashes, scalars, etc. To use the variable pointed to by a reference, you have to dereference the reference. This could also be what you were referring to by $$, because if we had a reference to a scalar, like this:

    my $string = "foo bar";
    my $ref    = \$string;
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then we could get access to the value "foo bar" by dereferencing $ref, like this:

    my $original = $$ref;
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$original now contains "foo bar".

So take a look at perlref and also at perlreftut, which might actually be a better place to start than perlref, depending on your experience.