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( $self ) ) {
    printf STDERR "\$self ( %s )  is\n", ref( $self );
}
open NEW, "> $new" or die "Couldn't open $new : $!";