$ perl -MIO::Zlib -e '$fh = IO::Zlib->new("test.gz", "wb"); print fileno($fh)'
AUTOLOAD(IO::Zlib=HASH(0x81e8b30))
IO::Zlib=HASH(0x81e8b30) IO::Zlib::FILENO 
main -e 1
Not a GLOB reference at /usr/lib/perl5/site_perl/5.8.0/IO/Zlib.pm line 566.