#!/usr/bin/perl

use warnings;
use strict;

opendir my $dir, '.' or die "opendir .: $!\n";
my $file;
my $count = 0;
while (defined($file = readdir($dir))) {
    # give yourself some progression feedback
    $count++;
    print "file $count ...\n" unless $count % 1000;

    # skip all files not begining with b
    next unless $file =~ /^b/;

    # if you've created directories, may need to skip them;
    # this will slow things down, so don't do so unless necessary
    next unless -f $file;

    # do something with the file

    rename $file, "b/$file" or die "rename $file b/$file: $!\n";
}