You'd need to use Win32::API or Inline C to do this. See here where is says:

To programmatically add or modify system environment variables, add them to the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\Environment registry key, then broadcast a WM_SETTINGCHANGE message with lParam set to the string "Environment". This allows applications, such as the shell, to pick up your updates.

No guarentees that a pre-existing shell instance would see your changes though. You might be able to inject code into the parent process to do it, but that's nasty and awkward.

If it helps, this is how I have changed the PATH env. var. using some other weird obscure language (OK, C++). Sorry it is not in Perl, I hope this is enough for you to get the gist:

#define ENVNAME "Path"
#define MAXLEN  1024

static void UpdatePath (char *szDir)
{
   char szPath [MAXLEN]={0};
   DWORD dwReturnValue;

   // Get previous value 
   if ( !GetEnvironmentVariable (ENVNAME, szPath, MAX_PATH) )
      ReportError ("GetEnvironmentVariable", GetLastError());

   // Update path 
   strcat (szPath, ";");
   strcat (szPath, szDir);

   // On W2K we need OSVERSIONINFOEX
   OSVERSIONINFO OsVer;
   OsVer.dwOSVersionInfoSize = sizeof (OsVer);
   GetVersionEx (&OsVer);

   if ( OsVer.dwPlatformId == VER_PLATFORM_WIN32_NT )
   {
      // Write to the user environment variables - 
      // only exists on WNT/W2K!

      HKEY hEnv;
      dwReturnValue = RegOpenKeyEx (HKEY_CURRENT_USER, 
                                    "Environment", 0, 
                                    KEY_SET_VALUE, 
                                    &hEnv) ;
      if ( dwReturnValue != ERROR_SUCCESS )
         ReportError ("RegOpenKeyEx", dwReturnValue);

      dwReturnValue = RegSetValueEx (hEnv, ENVNAME, 0, 
                                     REG_SZ, (BYTE*)szPath, 
                                     strlen(szPath)+1);

      if ( dwReturnValue != ERROR_SUCCESS )
         ReportError ("RegSetValueEx", dwReturnValue);

      RegFlushKey (hEnv);

      if ( hEnv ) RegCloseKey(hEnv);
   }
   
   // Tell the rest of the world (cmd line)

   if ( !SendMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE, 0,
                     (LPARAM) "Environment", SMTO_ABORTIFHUNG,
                     5000, &dwReturnValue) )
   {
      ReportError ("SendMessageTimeout", GetLastError());
   }

   // Tell our children 

   if ( !SetEnvironmentVariable (ENVNAME, szPath) )
      ReportError ("SetEnvironmentVariable", GetLastError());

}  // UpdEnv
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If the parent is a Perl script:

chomp( my @cmds = `child` );
foreach my $cmd (@cmds) {
   next unless /^set\s+(\S.*?)=(.*)/i;
   $ENV{$1} = $2;
}
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If the parent is a batch file:

for /f "usebackq delims=" %%f in (`child`) do %%f
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The child would output such as

echo set VAR=5
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print("set VAR=5\n");
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Update: Added second snippet.

Perhaps another approach would be to exec a new shell with the environment you want?

I'm not quite sure of the usefulness, but our IT department wants this for some sort of maintenance script. They say they have a compiled app that does it, but want to add more features than this app supports, so that is why I'm trying to do it.

- Tom