in reply to Printing with a Specific Scientific Notation Exponent

I think you need to roll your own for this. The following will get you started: 
use strict;
use warnings;

my $num = 0.05;
my $num_string = sprintf("%e", $num);
print "$num_string\n";
my($mantissa, $exponent) = split /e/, $num_string;
my $new_exp = $exponent + 4;
my $new_mantissa = $mantissa * 10**$new_exp;
print $new_mantissa, "e-04\n";
__DATA__
5.000000e-02
500e-04

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The right way to do this, of course, is in a subroutine, where you pass in your number along with the desired fixed exponent ('-4' in your example). In that case you need to change the '4' to a variable in the $new_exp calculation. You may also want to use sprintf to format the mantissa for your output (maybe you want 500.0 instead of just 500, for example).

Updated: You really need

$new_exp = $exponent - $fixed_exponent;

if you want to allow for an arbitrary fixed exponent. My example above applies only to negative values of the fixed exponent.

Never mind. I see now at the end of the OP that ysth supplied a better answer. :-)

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