$ perl -Mstrict -Mwarnings -le '
    my $x = 9**-10;
    print "$x";
    printf "%f" => $x;
    printf "%.15f" => $x;
    printf "%.31f" => $x;
    printf "%g" => $x;
    printf "%.15g" => $x;
    printf "%.31g" => $x;
'
2.86797199079244e-10
0.0000000.0000000002867970.00000000028679719907924413493062.86797e-102.86797199079244e-102.86797199079244134930566254989e-10$