1. is this a bug in $dbh->quote()

This cannot be answered without referring to the documentation. There it says:

As a special case, the standard numeric types are optimized to return $value without calling type_info.

The answer is therefore: no, this is expected documented behaviour, not a bug. You could however argue that it is a wart - having gone to the trouble of providing a quoting mechanism, it seems a bit unhelpful to exempt these types as a special case.

2. is there exists more correct way to insert numbers into SQL

All I can suggest is to create your own quoting method. Something like:

  sub quote_number {
    my($dbh, $value) = @_;
    if (defined $value) {
      # force to number
      return $value + 0;
    } else {
      return $dbh->quote($value);
    }
  }
[download]

Note that as written, that will give a warning (assuming they're turned on) on anything that doesn't look enough like a number; you may prefer to treat that more severely (die rather than warn) or less severely (don't warn at all); similarly the above code will permit decimals and negative numbers: if you want to be more restrictive than that you'll need to add some code to test either mathematically or with regexps, eg:

  sub quote_integer {
    my($dbh, $value) = @_;
    if (defined $value) {
      # pattern approach
      die "Not an integer" unless $value =~ /^[-+]?\d+\z/;
      # maths approach
      die "Not an integer" unless $value == int($value);
      return $value + 0;
    } else {
      return $dbh->quote($value);
    }
  }
[download]

Hugo