Tie::File: remove 5.005 compatibility code

- Ever since 5926ffc, Tie::File has been using warnings.pm and
  'our', making the code incompatible with 5.005 ('use warnings'
  wouldn't even compile anymore), so remove the old compatibility
  code.
- Update code for v5.8 (the "Unknown discipline" error from v5.6 was
  changed to "IO layers (...) unavailable" then).
- 'use constant' to declare constants.
- Remove redundant 'use strict "vars"' (the whole file is under 'use
  strict').

dist/Tie-File/Makefile.PL

@@ -21,6 +21,9 @@ WriteMakefile(
             MailingList => 'http://lists.perl.org/list/perl5-porters.html',
         },
     },
+    PREREQ_PM => {
+        'constant' => '1.03',
+    },
 );
 # generated by Makepmdist (mkpmdist) v1.01
 

dist/Tie-File/lib/Tie/File.pm

@@ -1,17 +1,15 @@
 package Tie::File;
 
-require 5.005;
-
 use strict;
 use warnings;
 
 use Carp ':DEFAULT', 'confess';
 use POSIX 'SEEK_SET';
 use Fcntl 'O_CREAT', 'O_RDWR', 'LOCK_EX', 'LOCK_SH', 'O_WRONLY', 'O_RDONLY';
-sub O_ACCMODE () { O_RDONLY | O_RDWR | O_WRONLY }
+use constant O_ACCMODE => O_RDONLY | O_RDWR | O_WRONLY;
 
 
-our $VERSION = "1.08";
+our $VERSION = "1.09";
 my $DEFAULT_MEMORY_SIZE = 1<<21;    # 2 megabytes
 my $DEFAULT_AUTODEFER_THRESHHOLD = 3; # 3 records
 my $DEFAULT_AUTODEFER_FILELEN_THRESHHOLD = 65536; # 16 disk blocksful
@@ -106,21 +104,14 @@ sub TIEARRAY {
   } elsif (ref $file) {
     croak "usage: tie \@array, $pack, filename, [option => value]...";
   } else {
-    # $fh = \do { local *FH };  # XXX this is buggy
-    if ($] < 5.006) {
-	# perl 5.005 and earlier don't autovivify filehandles
-	require Symbol;
-	$fh = Symbol::gensym();
-    }
     sysopen $fh, $file, $opts{mode}, 0666 or return;
     binmode $fh;
     ++$opts{ourfh};
   }
   { my $ofh = select $fh; $| = 1; select $ofh } # autoflush on write
-  if (defined $opts{discipline} && $] >= 5.006) {
-    # This avoids a compile-time warning under 5.005
-    eval 'binmode($fh, $opts{discipline})';
-    croak $@ if $@ =~ /unknown discipline/i;
+  if (defined $opts{discipline}) {
+    eval { binmode($fh, $opts{discipline}) };
+    croak $@ if $@ =~ /Unknown discipline|IO layers .* unavailable/;
     die if $@;
   }
   $opts{fh} = $fh;
@@ -1451,14 +1442,15 @@ package Tie::File::Cache;
 $Tie::File::Cache::VERSION = $Tie::File::VERSION;
 use Carp ':DEFAULT', 'confess';
 
-sub HEAP () { 0 }
-sub HASH () { 1 }
-sub MAX  () { 2 }
-sub BYTES() { 3 }
-#sub STAT () { 4 } # Array with request statistics for each record
-#sub MISS () { 5 } # Total number of cache misses
-#sub REQ  () { 6 } # Total number of cache requests 
-use strict 'vars';
+use constant {
+    HEAP  => 0,
+    HASH  => 1,
+    MAX   => 2,
+    BYTES => 3,
+    #STAT  => 4, # Array with request statistics for each record
+    #MISS  => 5, # Total number of cache misses
+    #REQ   => 6, # Total number of cache requests
+};
 
 sub new {
   my ($pack, $max) = @_;
@@ -1740,9 +1732,11 @@ sub delink {
 package Tie::File::Heap;
 use Carp ':DEFAULT', 'confess';
 $Tie::File::Heap::VERSION = $Tie::File::Cache::VERSION;
-sub SEQ () { 0 };
-sub KEY () { 1 };
-sub DAT () { 2 };
+use constant {
+    SEQ => 0,
+    KEY => 1,
+    DAT => 2,
+};
 
 sub new {
   my ($pack, $cache) = @_;