Thanks guys, I think I've got it now. I simplified the Dumper call and just pass refs around, which I think is what demerphq was talking about. The new _read and _write are:

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

sub _read { # still need to deal with race conditions
    my $file = shift;
    return undef unless -f $file;
    my $ret;
    unless ($ret = do $file) {
        warn "couldn't parse $file: $@" if $@;
        warn "couldn't do $file: $!"    unless defined $ret;
    }
    return $ret;
}
[download]

Now I have to read up on writing methods...