All the answers given so far are fine if there are 11* digits after the decimal point. But what if the mantissa is longer than 11 digits?:

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

Still truncated...

And what if there are considerably fewer digits:

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

Would the OP want all those extra zeroes?

One solution (admittedly not very elegant):

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

* Well, or 12 in this case.