#!/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'


##</code><code>##

#!/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;


##</code><code>##

perl -v

This is perl 5, version 18, subversion 4 (v5.18.4) built for MSWin32-x64-multi-thread