Do this for a quick list of what is and is not there:

print "<UL>\n";
for (sort keys %ENV) {
  print "<LI>$_ <STRONG>$ENV{$_}</STRONG>\n";
}
print "</UL>\n";
[download]

Be aware that this can be easily changed/faked, and that some browsers do not send anything at all (which could be what is happening here)