After reading Master of your Environment (NT), I started looking into OS detection. The faq says to use $^O but it only returns MSWin32 which is not all that helpful. So I turned to the docs for the Win32 module. After a minute or two (or five... I wan't counting), I had some working code in place that while still somewhat lacking, works.
Updated: I have done some further breakdown of the Win32 flavors... Please correct me if I have anything wrong here... I just whipped this up.
Yet to do:
coerce $build into a valid build number...
Note: I did not use $string as it is blank under Win98 and possibly other flavors as well. The docs say this about it: "An arbitrary descriptive string".
Updated once again: I looked into the Config module as well. However, $Config{osname} returns 'MSWin32' same as $^O and $Config{osvers} returns incorrect data (at least on Win 2k). 
#!/usr/bin/perl
use strict;
use warnings;
use Win32;

# this may be all some people need...
print "NT\n" if (Win32::IsWinNT());
print "9x\n" if (Win32::IsWin95());

# others may want to get more indepth...
my $os;
my ($string, $major, $minor, $build, $id) = Win32::GetOSVersion();
$os = "Unknown Win32s flavor" if ($id == 0);
if ($id == 1) {
    if ($minor == 90) {
        $os = "Win ME";
    }elsif ($minor == 10) {
        $os = "Win 98";
    }else{
        $os = "Win 95";
    }
}
if ($id == 2) {
    if ($major == 3) {
        $os = "Win NT 3.51";
    }elsif ($major == 4) {
        $os = "Win NT 4.0";
    }elsif ($major == 5) {
        if ($minor == 0) {
            $os = "Win 2k";
        }elsif ($minor == 1) {
            $os = "Win XP";
        }
    }else{
        $os = "Unknown NT flavor";
    }
}
print "$os, $major.$minor\n";

BEGIN {
    if ($^O !~ "Win32") {
        print "This is not a Win32 system.  Exiting...\n";
        exit;
    }
}

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In reply to Which flavor of Win32? by Mr. Muskrat

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