$fp1 =  1.9041105342991877e+258;;

$change = $fp1 - 1.9041105342991876e+258;                  ## note change from 7 to 6 in the last digit of the number being subtracted
printf "% 25.17g\n% 25.17g\n", $fp1, $change;;
  1.9041105342991886e+258
                        0                                  ## no change

$change = $fp1 - 1.9041105342991875e+258; 
printf "% 25.17g\n% 25.17g\n", $fp1, $change;;             ## note change from 7 to 5 in last digit
  1.9041105342991886e+258
                        0                                  ## no change

$change = $fp1 - 1.9041105342991874e+258;                  ## note change from 7 to 4 in last digit
printf "% 25.17g\n% 25.17g\n", $fp1, $change;;
  1.9041105342991886e+258
  2.1337646185215534e+242                                  ## A change!                                     ### 1 ###

## Now try the increment instead of decrement 
$change = $fp1 - 1.9041105342991878e+258;                  ## note change from 7 to 8 in last digit
printf "% 25.17g\n% 25.17g\n", $fp1, $change;;
  1.9041105342991886e+258
 -2.1337646185215534e+242                                  ## A change straight away!                      ### 2 ###

##### Note that whilst the sign of the change at ### 1 ### & ### 2 ### is different, the absolute value is the same.

##</code><code>##

$fp2 =  -8.2727285363069939e-293;;
printf "% 25.17g\n", $fp2;;
 -7.9999999999999948e-293                          ### WTF? ###