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 );
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;