this code should meet those requirements. at least, the mantissa width is fixed, the exponent width varies. it should be easy enough to modify this code to suit your needs, if you wish.

i used your test suite, too.

#!/usr/bin/perl
use strict;
use warnings;
$|++;


Main();
exit;


## adapted from code found at: http://www.cs.tut.fi/~jkorpela/c/eng.ht
+ml
sub eng
{
    my( $num, $digits )= @_;

    ## default to smallest number of digits allowing fixed width manti
+ssa (4)
    $digits= defined $digits && 3 < $digits 
        ? $digits 
        : 4;
    
    my $neg;

    if( 0 > $num )
    {
        $neg= 'true';
        $num= -$num;
    }

    0 == $num and return sprintf '+%.*fe+%s' => $digits - 1, $num, 0;
    my $exp= 0 != $num 
        ## perl's log() is natural log, convert to common log
        ? int( log($num) / log(10) )
        ## short-circuit: can't do log(0)
        : 0;

    ## tricky integer casting ahead...
    $exp= 0 < $exp
        ? int( ( int( $exp / 3 ) ) * 3 )
        : int( int( ( -$exp + 3 ) / 3 ) * -3 );

    $num *= 10 ** -$exp;

    if( 1000 <= $num )
    { 
        $num /= 1000;
        $exp += 3;
    }
    elsif( 100 <= $num )
    {
        $digits -= 2;
    }
    elsif( 10 <= $num )
    {
        $digits -= 1;
    }

    0 <= $exp 
        and $exp= '+' . $exp;
    
    return ( $neg ? '-' : '+' ) 
        . sprintf '%.*fe%s' => $digits - 1, $num, $exp;
}


sub Main 
{
    my $digits= 2;

    for my $exp ( -101..-98, -11, -10..11, 98..101 )
    {
        for my $sign ( '', '-' ) 
        {
            my $num= 0 + ( $sign . "5.555555555e" . $exp );
            printf "%-20s (%s)\n", $num, eng( $num, $digits );
        }
    }

    for my $exp (  -10..11 ) 
    {
        for my $sign (  '', '-'  ) 
        {
            my $num= 0 + ( $sign . "1e" . $exp );
            printf "%-20s (%s)\n", $num, eng( $num, $digits );
            printf "%-20s (%s)\n", 0, eng( 0, $digits )
                if  1 == $num;
        }
    }
}
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