Perl IO vs GLOB symbols

Hi,

Can someone please clarify how we are able to use both IO and GLOB symbols of a package variable interchangeably?

Please consider the following code:

open(FH,"myfile") || die "Unable to open file myfile:$@";
my $glob_var = *main::FH{GLOB};
my $io_var = *main::FH{IO};
print $glob_var "From glob\n";
print $io_var "From IO\n";
close FH;

Now 'myfile' contains,

From glob
From IO

Thanks,
Srini