Hello, I've just put this code

use IO::File;
print "Content-type: text/html\n\n";
print "Go..\n";
my $fh = IO::File->new_tmpfile();


if (defined $fh) {
    # Do something with the tmpfile
    print "Defined: $fh\n";
}
else {
    # Do something in memory
    print "Not defined\n";
}
[download]

in my site running IIS 4
and it seems no work.
If I run the code in a command window, the output is what I expect:

Go..
Defined: IO::File=GLOB(0x176f208)

I think is a permissions problem, the user which is executing that .pl is an anonymous user, and it has no permissions except in a couple of directories.
Hope this helps

Hopes