It surprised me too, since it's not what I got.

    $ perl6 --version
    This is Rakudo version 2018.03 built on MoarVM version 2018.03
    implementing Perl 6.c.
    $ perl6 -e 'print 9999999999999999.0-9999999999999998.0;print "\n";'
    2
    $

(Incidentally, I would have used "say" rather than "print" with an explicit newline.)