use strict;
use warnings;
use Math::MPFR qw(:mpfr);

print      FMOD(900719925474099.7, 2147483647.83), "\n";
print MPFR_FMOD(900719925474099.7, 2147483647.83), "\n";


sub FMOD { return $_[0] - int($_[0] / $_[1]) * $_[1]; }

sub MPFR_FMOD {
   # Do exactly what FMOD does - but on Math::MPFR objects, performing
   # the calculation at 200-bit precision.

   my $arg0 = Math::MPFR->new($_[0]); # 53 bit precision representation of $_[0]
   my $arg1 = Math::MPFR->new($_[1]); # 53 bit precision representation of $_[1]

   # Do the calculation at 200 bit precision for improved accuracy.
   Rmpfr_set_default_prec(200);

   my $ret = int($arg0 / $arg1);
   $ret *= $arg1;
   $ret = $arg0 - $ret;

   # Convert the 200-bit precision $ret to
   # a perl scalar (double), and return it
   my $nv = Rmpfr_get_d($ret, MPFR_RNDN);

   # Restore default precision back to the
   # original value of 53 before returning
   Rmpfr_set_default_prec(53);

   return $nv;
}

__END__

Outputs:
859064762.875
859064762.882