Okay, return as a string and then force perl to numerise it.

Trouble is, something gets lost in the transition:

use Math::BigFLoat;;

$n = Math::BigFloat->new( '3.1415926535897932384626433832795' );;

print $n;;
3.1415926535897932384626433832795

$d = 0+$n->bstr;;              ##### as a double

printf "%.17f\n", $d;;         ##### display to full precision
3.14159265358979310

$n -= $d;;                     ##### substract from the bigfloat

print $n;;                     ##### and display the (bigfloat) result
0.0000000000000032384626433832795
[download]

But:

    3.1415926535897932384626433832795
  - 3.1415926535897931000000000000000
    0.0000000000000001384626433832795 which is quite different to the 
+value from above!
    0.0000000000000032384626433832795
[download]

I expected some minor discrepancies, but not an order of magnitude difference.