This is slightly off-topic since the original poster was asking about fixed-point arithmetic, and not really significant digits, but....

It would be cool to have a perl library that would automatically apply the scientific "number of significant digits" rules.

my $n1=new Number::Scientific(5500,3); #5500 w/ 3 significatn digits
my $n2=new Number::Scientific(3.141592,7) # pi w/ 7 siginificant digit
+s
my $n3=$n1*$n2;
my $n4=$n2/$n1;
my $n5=$n1+$n2;
print "$n3 $n4 $n5\n";
[download]

would output 17300 0.000571 5500. Unfortunately I don't think any such beast exists.