#!/usr/bin/perl -w
use strict;
use File::Find;

# set these to the desired min and max
my $max_depth = 3;
my $min_depth = 3;

# find files
my @files;
find({ wanted => 
       sub {
           my $depth = tr!/!!; # count slashes to get depth
           return if $depth < $min_depth or $depth > $max_depth;
           push @files, $_;
       },
       no_chdir => 1 }, '.' );

# sort by mtime
@files = sort { (stat($a))[9] <=> (stat($b))[9] } @files;

# concat and print
foreach my $file (@files) {
    open FILE, $file or die "Can't open $_ : $!";
    while (<FILE>) { print }
}