But what happens when you want to pass a format or dirhandle to a subroutine instead of a filehandle.

Although globs are synonymous with filehandles they aren't really the same thing. It's just that perl will DWYM when you provide a reference to a glob with an IO reference in it

$FH = "a scalar";
@FH = qw/an array/;
%FH = (a => 'hash');
open(FH, "somefile.txt") or die("dang - $!");
for(qw/SCALAR ARRAY HASH IO/) {
  print $glob, "{$_} = ', *{$glob}{$_}, $/;
}
print while <$glob>;

__output__

*main::FH{SCALAR} = SCALAR(0x80fd44c)
*main::FH{ARRAY} = ARRAY(0x80fd464)
*main::FH{HASH} = HASH(0x80fd4a0)
*main::FH{IO} = IO::Handle=IO(0x80fd4dc)
foo
bar
baz
quux
[download]

So if you use a glob or a reference to a glob where a filehandle is expected and the glob doesn't contain a valid IO reference then it evaluates to undef e.g

$main::FH = 'a string';
print "nope" if not defined *FH{IO};

__output__

nope
[download]

HTH

_________
broquaint