And installing Math::decNumber is as easy as:

cpan -i Math::decNumber
[download]

There's no need to worry about first locating and downloading the decNumber library as it ships with the module - and the functionality is automatically built into the Math::decNumber dll/so as part of the build process.

All very easy, passes its test suite, and works well:

C:\> perl -MMath::decNumber="d_" -le "print d_(0.3) - d_(0.2) - d_(0.1
+);"
0.0

C:\>perl -MMath::decNumber="d_" -le "print sqrt(d_(3));"
1.732050807568877293527446341505872
[download]

No trig functions, unfortunately.

Cheers,
Rob