use strict;
use warnings;
my $Registry;
use Win32::TieRegistry (
  Delimiter => "/",
  TiedRef => \$Registry,
  "REG_EXPAND_SZ"
);
use Win32::API;

my $dir = "c:\\another_directory";

# Registry key for $ENV{PATH}
my $path_key = "LMachine/SYSTEM/CurrentControlSet/Control/Session Manager/Environment/Path";
die "Can't find Path registry key\n"
  unless exists $Registry->{$path_key};

my $path = $Registry->{$path_key};

$path .= ";$dir";
$Registry->{$path_key} = [ $path, REG_EXPAND_SZ ];

use constant HWND_BROADCAST   => 0xFFFF;
use constant WM_SETTINGCHANGE => 0x001A;
my $SendMessage = Win32::API->new("user32", "SendMessage", 
  [qw(N N P P)], "N");
my $result = $SendMessage
  ->Call(HWND_BROADCAST, WM_SETTINGCHANGE, 0, 'Environment');