my $file = $self->{Filename}; # This is detainted early on
my $new_ext = ".new";         # This cannot be tainted

my $new = $file . $new_ext;  # Not tainted
if ( is_tainted( $new ) ) {
    printf STDERR "%s is tainted by %s\n", $new,
	is_tainted( $file ) ? $file : $new_ext;
}
open NEW, "> $new" or die "Couldn't open $new : $!";