DB<115> $x1 = 2.56080e5
 => 256080

  DB<116> $x2 = 2.5816000000000003e5 
 => 258160                 # within error margin, ignored in normal display

  DB<117> printf '%.11f', $x2               
258160.00000000003         # forced into visibility, hence $x2 NOT an integer

  DB<118> $x2-$x1
 => 2080.00000000003       # error can't be ignored any more