Tie-ing hashes clobbers data

[Dave05]

Peace be upon the Monksters;

I have a simple (or so I thought) class ('FileHash') that ties a hash to a file, reading data into the hash upon instantiation, and writing the hash out to the file when DESTROYed.

I want to use the class for several hashes e.g.

my (%pwds, %boxes); 
tie (%pwds,  'FileHash', "$dir/pwds"); 
tie (%boxes, 'FileHash', "$dir/boxes");

but the second call to tie clears out any data that may have been read into the first hash and replaces it with the data from the second hash.

I just can't get my head around why this happens.

Here is the code for my class:

package FileHash;
use strict;
use warnings;
use Tie::Hash;
use Data::Dumper;
our @ISA = ("Tie::Hash");

sub TIEHASH {
   my ($class, $file) = @_;
   my $s = {}; 
   $s->{DATA} = _read($file) || {};
   $s->{FILE} = $file;
   bless $s, $class;
}

sub STORE    { $_[0]->{DATA}{$_[1]} = $_[2] }
sub CLEAR    { $_[0]->{DATA} = () }
sub FETCH    { $_[0]->{DATA}{$_[1]} }
sub FIRSTKEY { my $a = scalar keys %{$_[0]}; each %{$_[0]->{DATA}} }
sub NEXTKEY  { each %{$_[0]->{DATA}} }
sub EXISTS   { exists $_[0]->{DATA}{$_[1]} }
sub DELETE   { delete $_[0]->{DATA}{$_[1]} }
sub DESTROY  { _write ($_[0]) }

# ----- private subs -----
sub _write { # still need to deal with race conditions
   my %data = %{$_[0]->{DATA}};
   my $file =   $_[0]->{FILE} ;
   open (FH, "> $file") or die "Can't open $file for FileHash _write: 
+$!";
   print FH Data::Dumper->Dump([\%data], ['*data']); 
   close FH or die "Can't close $file for FileHash _write: $!";
}

sub _read { # still need to deal with race conditions
   no strict;
   my $file = shift;
   return undef unless -f $file;
   unless (my $ret = do $file) {
      warn "couldn't parse $file: $@" if $@;
      warn "couldn't do $file: $!"    unless defined $ret;
   }
   return \%data;
}

1;