would you get different output for all three if you used printf "%.10000g\n", unpack 'd', pack 'd', sqrt 2 ?

As is desirable, it's the same (and correct) output in all three cases:

D:\>perl -le "printf '%.10000g', unpack 'd', pack 'd', sqrt 2;"
1.4142135623730951454746218587388284504413604736328125
[download]

which, incidentally, is exactly the value of the double precision representation of sqrt 2.

Cheers,
Rob