There are only 6 globs slots and they are SCALAR, ARRAY, HASH, IO, FORMAT and CODE, so I can only assume by *glob{FILEHANDLE} you mean *glob{IO}. The *glob{IO} slot holds the reference to the IO object associated with a glob's filehandle (assuming it exists) e.g

open(FH, $0);

print "*FH IO slot holds - ", *FH{IO}, $/;

__output__

*FH IO slot holds - IO::Handle=IO(0x8107e4c)
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Not meaning to plug (but will anyway) I'd recommend Of Symbol Tables and Globs for more info on globs and glob slots.

How/where/why it is used? Please give me example.

Accessing glob slots directly is very rarely done, and when it is, it's usually for doing low-level code like writing Exporter type code. Here's a simple demo of such code (if somewhat contrived)

sub import {
  my $pkg = caller;

  ## import only filehandles
  *{"$pkg\::$_"} = *{__PACKAGE__."::$_"}{IO}
    for keys %{__PACKAGE__."::"};
}
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HTH

_________
broquaint

update: s/DIR/CODE/ + added example