How do Perl, Wolfram ans Soup get the "right" (ahaha...) answer ? (I'm not familiar with these langages)

Of course for others it should "fixable" where needed:

  ~ python3
  Python 3.6.7 (default, Oct 22 2018, 11:32:17) 
  [GCC 8.2.0] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  >>> from decimal import Decimal
  >>> Decimal('9999999999999999.0') - Decimal('9999999999999998.0')
  Decimal('1.0')