$ perl -Mstrict -Mwarnings -le '
    my $x = 9**-10;
    print "$x";
    print sprintf "%f" => $x;
    print sprintf "%.15f" => $x;
    print sprintf "%.31f" => $x;
    print sprintf "%g" => $x;
    print sprintf "%.15g" => $x;
    print sprintf "%.31g" => $x;
'
2.86797199079244e-10
0.000000
0.000000000286797
0.0000000002867971990792441349306
2.86797e-10
2.86797199079244e-10
2.86797199079244134930566254989e-10