I'm having trouble figuring out why

sub data_for {
  binmode($_[0]);
  return <$_[0]>;
}
[download]

doesn't return binary data read from a filehandle argument. I managed to get this subroutine working by instead doing

sub data_for {
  binmode($_[0]);
  read( $_[0], my $data, 100_000_000 );
  return $data;
}
[download]

I've skimmed through perlsub perlref and perlfaq5 but can't seem to explain why my first attempt didn't work