my $long= unpack "P4", pack "L", $addr;
[download]

my $long= unpack "P4", pack "I", $addr;
[download]

Unpack wants a packed pointer, not a number. Unpack takes a string. If you give it a number, Perl will format the number into a string in decimal and pass that to unpack. Unpack will pull the first 4 characters (bytes) out and treat them like a (void *) and try to dereference that.

Update: The replying Anonymonk isn't particularly helpful and the pack docs aren't exactly clear on this point (that I saw) but I suspect that "I" gives an IV which is defined to be big enough to hold a pointer.