A Unix directory may be implemented as a file with one entry per file, but that doesn't necessarily equate to one line of a text file. The precise format of the file is a low-level implementation detail and I doubt it would be the same on all variants of Unix. If I type cat . on HP-UX I just get gibberish, containing recognisable filenames plus a load of binary stuff - and no line-ends.

Is there a reason why the diamond operator in Perl doesn't call readdir for directories, the same way it calls readline for regular files ?

I suppose they could have done that, but I can't comment on why they didn't. It would have to be able to detect whether it was working on a filehandle created by open or a dir handle created by opendir, but I guess that's not unreasonable.

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s^^unp(;75N=&9I<V@`ack(u,^;s|\(.+\`|"$`$'\"$&\"\)"|ee;/m.+h/&&print$&

Orthagonality with the C opendir(2) and friends would be my guess. If it really mattered to you you probably could implement a tied filehandle that provided its own map from readline to readdir without too much trouble.

Update: Yeah, not too much trouble at all.

#!/usr/bin/perl
use strict;

package Tie::Dirfile;
use IO::Dir ();

sub TIEHANDLE {
  my $class = shift;
  my $self = {};

  $self->{_dir} = shift;
  $self->{_handle} = IO::Dir->new( $self->{_dir} );
  
  return bless $self, $class;
}

sub READLINE { return shift()->{_handle}->read; }
sub CLOSE { return shift()->{_handle}->close( ); }

package main;

local( *DH );
tie *DH, 'Tie::Dirfile' => "/bin"
  or die "Can't create Tie::Dirfile for /bin: $!\n";

my $i = 0;
print $i++, ": ", $_, "\n" while( <DH> );

close( DH );

exit 0;

__END__
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They're not considered the same.

We have the higher-level opendir(3), readdir(3), closedir(3) etc functions in libc exactly because directories are not treated uniformly between UNIX systems.

For example, in Linux systems, you can not actually read directories. You can open them, but if you try to read them, you get EISDIR. The way readdir reads them is the SysV function getdents (and its lfs equivalent, getdents64).