You'll probably want to md5 the name to more evenly distribute the prefix letters, then use one or two of the hex characters at each level until the final directories are small enough. 
use Digest::MD5 qw(md5_hex);
my $name = "123456.jpg";
my $path = md5_hex($name);
$path =~ s#^(.)(.)(.).*#$1/$2/$3/$name#;

[download]
Three hex characters will distribute 400K files into 100 files per directory. Cache::FileCache uses a similar scheme.

-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.

In reply to •Re: Need directory scheme to store 400,000 images by merlyn
in thread Need directory scheme to store 400,000 images by markjugg

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