Because you are not using strict or warnings, you cannot see that DACL_SECURITY_INFORMATION is not defined. This results in $pSecDesc containing the wrong data, which cannot be decoded.
Adding this line populates $pSecDesc correctly:

  use Win32 qw( DACL_SECURITY_INFORMATION );

[download]

You still need to decode the Security Descriptor; this (thrown together, barely tested) program may help: 

use strict;
use warnings;
use Data::Dumper; $Data::Dumper::Useqq = 1; $| = 1;

my $host = '127.0.0.1';
my $reg_key = 'SOFTWARE\\7-zip';

use Win32 qw( DACL_SECURITY_INFORMATION );
use Win32API::Registry 0.21 qw( :ALL ); 

# Get the Security Descriptor
my $reg_api; 
RegConnectRegistry( $host, HKEY_LOCAL_MACHINE, $reg_api ) or die $^E;

my $key;
RegOpenKeyEx( $reg_api, $reg_key, 0, KEY_READ, $key )  or die $^E;

my $pSecDesc;
RegGetKeySecurity( $key, DACL_SECURITY_INFORMATION, $pSecDesc ) or die
+ $^E;

# Print the raw DACL.
print "\n", Dumper $pSecDesc;

# Decode the Security Descriptor
use Win32::API;
use constant SDDL_REVISION_1  => 0x1;

my $ConvertSDToString = Win32::API->new(
  'ADVAPI32', 
  'ConvertSecurityDescriptorToStringSecurityDescriptor',
  ['P', 'N', 'N', 'P', 'N'],
  'N',
);

my $ptr_strSDDL = pack 'L', 0; # DWORD;
my $ObjSD = $ConvertSDToString->Call(
  $pSecDesc, 
  SDDL_REVISION_1, 
  0xF, 
  $ptr_strSDDL,
  0,
) or die $^E;

my $strSDDL = unpack 'p', $ptr_strSDDL;

# Print the decoded DACL.
print "\n", Dumper $strSDDL;

[download]

In reply to Re: Win32API::Registry::RegGetKeySecurity how to unpack the structure of $pSecDesc? by Util
in thread Win32API::Registry::RegGetKeySecurity how to unpack the structure of $pSecDesc? by Sioln

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