That's the nature of floating point values. They are just (very accurate) approximations. See this Wikipedia article for more.

For example

$ perl -e '$a+=0.1 for 1..10; print $a - 1'
-1.11022302462516e-16
[download]

See also PerlFAQ4 - Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?

Update: Added example code
Update^2: Eventually found the FAQ