UPDATE:
This issue was caused by my ignorance of my Perl's pointer size (8 bytes in this case). Packing the pointer into an integer of the correct size "fixes" the issue I was experiencing. Thank you tye and ikegami for your help!

(It is obvious now in hindsight, but maybe the Perldocs for pack/unpack could be updated to mention the 'p/P' templates and Perl's pointer size?)

Good evening!

I am trying to more clearly learn how pack/unpack works with values returned from Win32::API, specifically pointers and how to dereference them. I have a small example program that calls a "cat" function in a "test.dll" that takes two strings and returns them concatenated together.

Setting the output parameter in the function template to char* works as expected:

#!/usr/bin/perl
use 5.018;
use strict;
use warnings;
use Win32::API;
use Data::Dumper;

$Win32::API::DEBUG = 1;

my $method = new Win32::API::More('test.dll', 'char* cat(char* a, char
+* b)');
if( !defined $method ) {
    die "Can't import API [cat]: $^E\n";
}

my $return = $method->Call('my ', 'potato');

print Dumper $return; # prints 'my potato'
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Setting the output parameter to int in the function template returns a valid pointer (I can peek at that memory address and see the concatenated string), and I can dereference it with various combinations of pack/unpack, but I do not know why some work:

#!/usr/bin/perl
use 5.018;
use strict;
use warnings;
use Win32::API;
use Data::Dumper;

$Win32::API::DEBUG = 1;

my $method = new Win32::API::More('test.dll', 'int cat(char* a, char* 
+b)');
if( !defined $method ) {
    die "Can't import API [cat]: $^E\n";
}

my $return = $method->Call('my ', 'potato');

print Dumper $return; # pointer address: 42952784

# works as expected: 'my potato'
my $x = unpack 'p', pack 'J', $return;
print Dumper $x;

# works as expected: 'my potato'
my $x = unpack 'p', pack 'Q', $return;
print Dumper $x;

# does not work as expected: undef
my $x = unpack 'p', pack 'I', $return;
print Dumper $x;

# does not work as expected: undef
my $x = unpack 'p', pack 'L', $return;
print Dumper $x;

# does not work as expected: undef
my $x = unpack 'p', pack 'N', $return;
print Dumper $x;

# Update: I made a typo in my original post.
# This (obviously) will not work on little-endian machines
#my $x = unpack 'p', pack 'N2', $return;
#print Dumper $x;

# works(??): 'my potato'
#my $x = unpack 'p', pack 'V2', $return;
#print Dumper $x;

# works(??): 'my potato'
my $x = unpack 'p', pack 'IN', $return;
print Dumper $x;
[download]

What exactly is going on with the different combinations of pack/unpack?

Perl info:

perl -v

This is perl 5, version 18, subversion 4 (v5.18.4) built for MSWin32-x
+64-multi-thread
[download]

Thank you for your time