The size of a pointer varies between different builds of Perl (even on the same system). The size of a pointer (in bytes) is available from

perl -V:ptrsize
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and

perl -E"use Config qw( %Config ); say $Config{ptrsize};"
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There is no pack format that's guaranteed to always produces exactly the size of a pointer on all systems. That means you need to determine the correct format from the aforementioned variable if you want a portable solution:

use Config qw( %Config );

my $ptr_size = $Config{ptrsize};
my $ptr_format =
   $ptr_size == 8 ? 'Q' :
   $ptr_size == 4 ? 'L' :
   die("Unrecognized pointer size");

my $s = unpack 'p', pack $ptr_format, $addr;
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You have a pointer size of 8 bytes, so "Q" is the appropriate format for you.

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To my knowledge, Windows only runs on little-endian systems. On a little-endian system, if you use an integer type that's too large, you'll simply add trailing zeros that will be ignored by unpack 'p'.

>perl -wE"say unpack 'p', pack('P', 'abc')"
abc

>perl -wE"say unpack 'p', pack('P', 'abc').qq{\0\0\0\0\0\0\0\0}"
abc
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As such, you can safely use "J" on such a system since the IV and UV types are guaranteed to be large enough to hold a pointer.

my $s = unpack 'p', pack 'J', $addr;
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Some types should be avoided since they have unpredictable size: "I".

Some types should be avoided since they don't necessarily use native byte order: "N", "n", "V", "v".

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# works(??): 'my potato'
my $x = unpack 'p', pack 'N2', $return;
print Dumper $x;
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huh??? Aren't you on an x86 machine? That's a little-endian machine. On a little-endian system such as yours, "J" and "N2" will give different result, so they can't possibly both work.

>perl -wE"say sprintf '%vX', pack 'J', 0x0000000012345678"
78.56.34.12.0.0.0.0
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>perl -wE"say sprintf '%vX', pack 'N2', 0x0000000012345678"
12.34.56.78.0.0.0.0
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# works(??): 'my potato'
my $x = unpack 'p', pack 'IN', $return;
print Dumper $x;
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You got lucky there.

>perl -wE"say sprintf '%vX', pack 'J', 42952784"
50.68.8F.2.0.0.0.0

>perl -wE"say sprintf '%vX', pack 'IN', 42952784"
50.68.8F.2.0.0.0.0
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But it would have failed for larger addresses since your "I" only produces 4 bytes.

>perl -wE"say sprintf '%vX', pack 'J', 0x0123456789ABCDEF"
EF.CD.AB.89.67.45.23.1

>perl -wE"say sprintf '%vX', pack 'IN', 0x0123456789ABCDEF"
EF.CD.AB.89.0.0.0.0
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When packing, there's no difference between corresponding signed and unsigned integer formats. I only mentioned unsigned pack formats above, but everything I said applies to the signed formats too.