Hmm. There seem to be two interacting problems here.

1. exp doesn't appear to be overloaded by either bignum or Math::BigFloat?

   #!perl -slw
   use strict;
   use overload;
   #use bignum;
   use Math::BigFloat;
   
   printf "exp(-7e-17): %.17f\n", my $exp = exp( Math::BigFloat->new( '-7
   +e-17' ) );
   
   eval{ print overload::Overloaded( $exp ) } or warn $@;
   print overload::StrVal( $exp );
   
   printf "1-exp(-7-e17): %.17f\n", Math::BigFloat->new("1") - $exp;
   __END__
   C:\test>junk7
   exp(-7e-17): 0.99999999999999989
   Can't call method "can" without a package or object reference at c:/Pe
   +rl/lib/overload.pm line 54.
   1
   1-exp(-7-e17): 0.00000000000000000
   [download]

2. And with bignum enabled, Math::BigFloat seems to forget how to do math? At least if the math involves one BigMath object and one normal perl number.

   #!perl -slw
   use strict;
   use overload;
   use bignum;
   use Math::BigFloat;
   
   printf "exp(-7e-17): %.17f\n", my $exp = exp( Math::BigFloat->new( '-7
   +e-17' ) );
   
   eval{ print overload::Overloaded( $exp ) } or warn $@;
   print overload::StrVal( $exp );
   
   printf "1-exp(-7-e17): %.17f\n", Math::BigFloat->new("1") - $exp;
   
   __END__
   C:\test>junk7
   exp(-7e-17): 0.99999999999999989
   Can't call method "can" without a package or object reference at c:/Pe
   +rl/lib/overload.pm line 54.
   1
   1-exp(-7-e17): 1.00000000000000000
   [download]

All of which makes me glad that I rarely need the accuracy beyond 53-bits and when I do, 64-bit ints suffice.