Errno - System errno constants
use Errno qw(EINTR EIO :POSIX);
Errno defines and conditionally exports all the error constants defined in your
system errno.h include file. It has a single export tag, :POSIX, which will export all 
POSIX defined error numbers.
Errno also makes %! magic such that each element of %! has a non-zero value only if $! is set to that value, eg
    use Errno;
    
    unless (open(FH, "/fangorn/spouse")) {
        if ($!{ENOENT}) {
            warn "Get a wife!\n";
        } else {
            warn "This path is barred: $!";
        } 
    } 
Graham Barr <gbarr@pobox.com>
Copyright (c) 1997-8 Graham Barr. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.