I've now tested this on my machine with an ATI OpenGL driver installed, and there are some more steps necessary to go through before glGetString will work, as the relevant OpenGL ICD(s) must be loaded, which doesn't happen until after OpenGL is initialized. After some short magic with Inline::C and some browsing of the MSDN, the below code is a very convoluted way to print out ATI Technologies Inc. on my machine. I've left in some warnings and didn't golf down the code, but in theory this is what should work everywhere for Win32.

use strict;
use Win32::API;

my $library = 'OpenGL32';

sub loadFunction {
  my $code = Win32::API->new(@_);
  
  die "Couldn't load $_[1] from library $_[0]: $! / $^E"
    unless $code;
  
  no strict 'refs';
  *{$_[1]} = sub { $code->Call(@_) };
};

loadFunction('USER32', 'GetDC', 'L','L');
loadFunction('GDI32', 'ChoosePixelFormat', 'LP','L');
loadFunction('GDI32', 'SetPixelFormat', 'LLP','L');
loadFunction($library, 'wglCreateContext', 'L','L');
loadFunction($library, 'wglMakeCurrent', 'LL','L');
loadFunction($library, 'glGetString', 'L','P');

my $hDC = GetDC(0) or die "Couldn't get the Device Context";
warn "Device context $hDC";

my $pfd = pack "ssVccccccccccccccccccccVVV", (40,1,0x25,24,0,0,0,0,0,0
+,0,0,0,0,0,0,0,0,16,0,0,0,0,0,0,0,);
my $ofs = pack "p", $pfd;
my $pfd_num = ChoosePixelFormat($hDC,$pfd);
die "Didn't get a (solid) pixel format: $pfd_num" unless $pfd_num;
SetPixelFormat($hDC,$pfd_num,$pfd)
  or die "Couldn't set the proper pixel format";

my $hRC = wglCreateContext($hDC) or die "Couldn't create the GL Render
+ing Context";
wglMakeCurrent($hDC,$hRC) or die "Couldn't activate the Rendering Cont
+ext";
print glGetString(0x1F00);
[download]

I'm not sure what idea the MESA GL libraries subscribe to, but in theory, they should work the same, if they are the only installed ICD.