printf "dp:%u : %35.*f\n", $_, $_, 3.1415926535897932384626433832795 for 14 .. 20;;
dp:14 :                    3.14159265358979
dp:15 :                   3.141592653589793
dp:16 :                  3.1415926535897931
dp:17 :                 3.14159265358979310
dp:18 :                3.141592653589793100
dp:19 :               3.1415926535897931000
dp:20 :              3.14159265358979310000

##</code><code>##

[0] Perl> printf "dp:%u : %.*f\n", $_, $_, 123e-308 for 300 .. 320;;
dp:300 : 0.000 {300 zeros ellided} 000000000000
dp:301 : 0.000 {300 zeros ellided} 0000000000000
dp:302 : 0.000 {300 zeros ellided} 00000000000000
dp:303 : 0.000 {300 zeros ellided} 000000000000000
dp:304 : 0.000 {300 zeros ellided} 0000000000000000
dp:305 : 0.000 {300 zeros ellided} 00000000000000000
dp:306 : 0.000 {300 zeros ellided} 000000000000000001
dp:307 : 0.000 {300 zeros ellided} 0000000000000000012
dp:308 : 0.000 {300 zeros ellided} 00000000000000000123
dp:309 : 0.000 {300 zeros ellided} 000000000000000001230
dp:310 : 0.000 {300 zeros ellided} 0000000000000000012300
dp:311 : 0.000 {300 zeros ellided} 00000000000000000123000
dp:312 : 0.000 {300 zeros ellided} 000000000000000001230000
dp:313 : 0.000 {300 zeros ellided} 0000000000000000012300000
dp:314 : 0.000 {300 zeros ellided} 00000000000000000123000000
dp:315 : 0.000 {300 zeros ellided} 000000000000000001230000000
dp:316 : 0.000 {300 zeros ellided} 0000000000000000012300000000
dp:317 : 0.000 {300 zeros ellided} 00000000000000000123000000000
dp:318 : 0.000 {300 zeros ellided} 000000000000000001230000000000
dp:319 : 0.000 {300 zeros ellided} 0000000000000000012300000000000
dp:320 : 0.000 {300 zeros ellided} 00000000000000000123000000000000

##</code><code>##

use Math::BigFloat;;

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

$d = 0 + $n->bstr;;

printf "%.17f\n", $d;;
3.14159265358979310

$bfd = Math::BigFloat->new( sprintf "%.17f", $d );;
print $bfd;;
3.1415926535897931

$n -= $bfd;;
print $n;;
0.0000000000000001384626433832795

##</code><code>##

    3.14159265358979310
  + 0.0000000000000001384626433832795
  = 3.1415926535897932384626433832795 == 3.1415926535897932384626433832795 (the input)