printf "%.11e\n", 0.99982928833123;
# 9.99829288331e-001

##</code><code>##

printf "%.11e\n", 0.999;
# 9.9980000000e-001

##</code><code>##

my $dec = 0.99982928833123;
my $len = length( ( split /\./, $dec )[1] );
printf "%.${len}e\n", $dec;
# 9.99829288331230e-001