sub insert_sep {
    my ( $number, $separator ) = @_;
    return if ! defined $number;
    $separator //= ',';
    return $number if $number =~ /\Q$separator\E/;
    return $number if $number !~ /^([+-]?\d{4,})(\D\d+)?\z/;
    my ( $n1, $n2 ) = ( $1, $2 );
    $n1 =~ s/(\d)(?=(?:\d{3})+\b)/$1$separator/g;
    return $n1 . $n2 if $n2;
    return $n1;
}