use SysInfo qw(RunUnder);
if (RunUnder 'Win32') {
 if (RunUnder 'Win2000+') {
  print "Running under Win2000 or WinXP\n";
 } else {
  print "Running under some older or more stripped down Windows\n";
} elsif (RunUnder 'Unix') {
 if (RunUnder 'BSD') {
  print "Runnind under BDS Unix\n";
 } else {
  print "Running under some other Unix\n"
 }
} else {
 print "GOK what are you using\n";
}