-1 -> <1K
0 -> 
1 -> <1K
10 -> <1K
100 -> <1K
511 -> <1K
512 -> 1K
513 -> 1K
999 -> 1K
1000 -> 1K
1023 -> 1K
1024 -> 1K
1025 -> 1K
10000 -> 10K
100000 -> 98K
1000000 -> 977K
10000000 -> 9766K
100000000 -> 95M
1000000000 -> 954M
10000000000 -> 9537M
100000000000 -> 93G
1000000000000 -> 931G
10000000000000 -> 9313G
100000000000000 -> 93132G
1000000000000000 -> 931323G
[download]

-1 gives an odd (arguable?) result, 0 gives nothing, and anything from 512 to 1023 is 1K, rather than <1K.

Suppose I want to extend it to teras and petas... and beyond? Can you see the redundancy in your code? How could you factor that out? If you did, it would be easier to extend.

BTW, my harness looks like this (ugly, but it gets the job done).

for my $amount( qw/-1 0 1 10 100 511 512 513 999 1000 1023 1024 1025
    10000    
    100000    
    1000000    
    10000000    
    100000000    
    1000000000    
    10000000000    
    100000000000    
    1000000000000    
    10000000000000    
    100000000000000    
    1000000000000000/ )      
{
    print "$amount -> ", amount($amount), "\n";
}
[download]

Ok, enough blabbering, here's how I would do it. Note that my routine produces different output. I think that is due to the way you employ 512 in your algorithm. I don't think that's necessary, but I'm not paying close attention.

sub amount {
    my $num = shift;
    return ''    if $num < 0;
    return '0K'  if $num == 0;
    return '<1K' if $num < 1024;

    my @suffix = qw/K M G T P/;
    my $offset = -1;

    while( $num >= 1024 and $offset < scalar @suffix ) {
        ++$offset;
        $num = int( $num / 1024 );
    }
    return "$num$suffix[$offset]";
}
[download]

Extending this routing is as simple as increasing the @suffix array, which is as it should be. Note to readers: this routine, like the original both run cleanly under strict and warnings.

If you're curious, after peta comes exa (E), zetta (Z) and yotta(Y). Man, think of all the mp3s you could store on a yottabyte, of course, the backups would kill you. Just don't get me started on kibibytes and mebibytes.