but not an order of magnitude difference

The base 2 representation of 3.1415926535897932384626433832795 is:
0.1100100100001111110110101010001000100001011010001100001000110100.....e2

Round that to 53 bits:
0.11001001000011111101101010100010001000010110100011000e2

Convert that value back to base 10:
3.14159265358979311599796...

I don't know how perl and Math::BigFloat arrive at the values they do, but I do know that off-by-one (or more) ULP errors are not unusual in perl.
Also, you've stretched the requested precision to 17 decimal digits, which might be a contributing factor - need to think more about that.

If you want reliable rounding and accuracy with floating point values, use Math::MPFR.

Cheers,
Rob

In reply to Re^3: Math::BigFloat to native double? by syphilis
in thread Math::BigFloat to native double? by BrowserUk

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