c:\>perl -MMath::BigFloat 
-wle"print Math::BigFloat->new(1) - Math::BigFloat->new( .999999999999999 )"
0.000000000000001

c:\>perl -MMath::BigFloat 
-wle"print Math::BigFloat->new(1) - Math::BigFloat->new( .9999999999999999 )"
0

c:\>perl -MMath::BigFloat 
-wle"print Math::BigFloat->new(1) - Math::BigFloat->new('.9999999999999999')"
0.0000000000000001

c:\>perl -MMath::BigFloat 
-wle"print Math::BigFloat->new(1) - Math::BigFloat->new('.99999999999999999999999')"
0.00000000000000000000001