I'd use Win32::TieRegistry.

#!/perl/bin/perl
use strict;
use warnings;
use Win32::TieRegistry;

my $host = shift || '';
my $hklm = $Registry->Connect($host, "HKEY_LOCAL_MACHINE", { Access =>
+ 'KEY_READ' } ) or die $^E;
my $info = $hklm->{"SOFTWARE\\Microsoft\\Windows NT\\CurrentVersion"};
my $ver = $info->{"CurrentVersion"};
my $sp = $info->{"CSDVersion"};
my $os = $hklm->{"SYSTEM\\CurrentControlSet\\Control\\ProductOptions\\
+ProductType"};
print "Remote OS: $os, Version: $ver, $sp\n";
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NOTE: This will not work for Win9x or WinME.

Update: You could also use Win32::MachineInfo.