# -*- Perl -*-

use strict;
use warnings;

package PerlIO::via::MyHeader;
our $VERSION = '0.01';

use Carp ();

sub PUSHED { 
    my ($class,$mode)=@_;
    Carp::croak __PACKAGE__, " can be used only for writing to file."
      unless $mode =~ /w/;
    bless \(my $x), $class;
}

my %done;

sub WRITE {
    my ($obj,$buf,$fh)=@_;
    print $fh ($done{$obj}++ ? '' : <<'    HEADER'), $buf;
START_OF_FILE
      
DATEFORMAT=YYYYMMDD
DECIMALSEPARATOR=.
    HEADER
}

1;

__END__

##</code><code>##

#!/usr/bin/perl

use strict;
use warnings;
use lib '.';
use PerlIO::via::MyHeader;

open my $fh1, '>:via(MyHeader)', "test1" or
  die "Can't open 'test1': $!\n";
open my $fh2, '>:via(MyHeader)', "test2" or
  die "Can't open 'test2': $!\n";

print $fh1 "foo\n";
print $fh1 "bar\n";
print $fh2 "foo\n";
print $fh2 "bar\n";

__END__

##</code><code>##

C:\temp>perl testmh.pl

C:\temp>diff test1 test2

C:\temp>cat test1
START_OF_FILE

DATEFORMAT=YYYYMMDD
DECIMALSEPARATOR=.
foo
bar