in reply to
4253.95 + 0.9 = 4254.84999999999 (need help to interpret internals)

If you care about your money, use Math::BigFloat, as dragonchild suggested.

As others have said, the problem is that floating point binary can't represent most decimal fractions exactly. It's analogous to the way that 1/3 is a repeating decimal (0.3333...). 0.95 and 0.9 are both repeating binary numbers. As can you see, it goes both ways: even 1/3 in trinary notation does not repeat: 0.1.

The solution is to use "fixed point" or "arbitrary precision" decimal arithmetic. This is usally implemented internally as an integer (or byte array) with a scale. I'm not sure how Math::BigFloat does its job, but it is the standard module for arbitrary precision arithmetic in Perl.