How do I perform decimal arithmetic in Perl?

(NOTE: There is Math::Currency but it is currently broken).

Use Math::BigFloat to represent numbers as arbitrary precision objects.

use Math::BigFloat;

print Math::BigFloat->new(0.1) +
      Math::BigFloat->new(0.1) +
      Math::BigFloat->new(0.1) == Math::BigFloat->new(0.3);

You can do this automatically with bignum...

use bignum;

print 0.1 + 0.1 + 0.1 == 0.3;

BUT! the magic only works on numbers. If you try to add strings together it won't work, the magic comes too late. You have to explicitly force them to be numbers. To numify a string you can add 0 to the string, like $a += 0. Or you can force an equation to be done as bignums by starting with 0 + and it will cascade down the line.

use bignum;

$a = "0.1";
$b = "0.1";
$c = "0.1";
$d = "0.3";

# False
print $a + $b + $c == $d;

# True
print 0 + $a + $b + $c == $d;

Two caveats.

First, this all comes at a heavy performance cost. Not only for doing arbitrary precision math, but for all the methods and overloading magic. Benchmark it to see if this is acceptable. Fortunately bignum only upgrades numbers in its scope, not the whole program. It's also safe to use those numbers outside of bignum's scope, any math done with them will also be upgraded.

Second, Decimal will preserve significant figures. Math::BigFloat will not.