The core of PDL is written in the C programming language (see PDL::Internals). Fortran is only used in some libraries such as PGPLOT (used for plotting) and Slatec (used for manipulating matrices, calculate Fast Fourier Transforms, fit data using polynomials, interpolate data, etc.). About the piddles' data types, here is a table that will show you the data types already defined (in the file Basic/Core/Types.pm.PL):
PDL Type Real C Type
byte unsigned char
ushort unsigned short
long long int
longlong long long int
Note that you can add new types as explained in the PDL documentation. You might also want to read the documentation on PDL::PP to learn how to add your own routines.
In short, I believe the computational accuracy of PDL to be closer to that of C than to the computational accuracy of Fortran.