use warnings;
use strict;
use POSIX;

for my $num (0, -0.1, 0.1, 999.99, 1000, -999.99, -1000, 1.234e34, 1.234e-34){
        printf "%-10s %-10s\n", $num, asEng ($num);
    }


sub asEng {
    my ($num) = @_;
    
    return '0' if +$num eq '0';

    my $sign = ($num < 0) ? '-' : '';

    $num = abs $num;

    my $e    = floor( log($num) / log(1000) );
    my $mult = 1000**$e;

    $e *= 3;
    $num = $num / $mult;
    return "$sign${num}e$e";
}

##</code><code>##

0          0         
-0.1       -100e-3   
0.1        100e-3    
999.99     999.99e0  
1000       1e3       
-999.99    -999.99e0 
-1000      -1e3      
1.234e+034 12.34e33  
1.234e-034 123.4e-36