------  Error: off
--- Reset: off
$! 0: 

--- Reset: on
$! 0: 

--- Reset: off
$! 17: File exists

--- Reset: on
$! 0: 

--- Reset: off
$! 17: File exists

--- Reset: on
$! 0: 


------  Error: on
--- Reset: off
$! 17: File exists

--- Reset: on
$! 9: Bad file descriptor

--- Reset: off
$! 9: Bad file descriptor

--- Reset: on
$! 9: Bad file descriptor

--- Reset: off
$! 9: Bad file descriptor

--- Reset: on
$! 9: Bad file descriptor

##</code><code>##

use strict;
use warnings;

$|=1;
my $dh;


sub tst {
  my ($mk_error, $reset_errno)=@_;
  
  mkdir "/tmp/bla";
  opendir $dh, "/tmp/bla" or warn "$!";

  rmdir "/tmp/bla" if $mk_error;


  $!=undef if $reset_errno;
  
  my $a= scalar readdir($dh);
  
  print "\n\$! ",$!+0,": $!\n" ;
}

for my $mk_error (0,1) {
  print "\n\n------  Error: ", (qw(off on))[$mk_error] ;
  for my $reset_errno (0,1,0,1,0,1) {
    print "\n--- Reset: ", (qw(off on))[$reset_errno] ; 
    tst($mk_error,$reset_errno) ;
  }
}


##</code><code>##

 perl -version

This is perl, v5.10.0 built for i486-linux-gnu-thread-multi