use strict;
use warnings;

use File::Find; # for traversing directories with find()

my %b_paths; # hash holding relative paths of files of the 'b' directory
my @dirs = ('a', 'b'); # directories to investigate

my $dirselect = 1; # use second @dirs entry
# Populate the hash with paths from the second @dirs entry directory.
# Use the relative paths as keys and 'undef' as value.
chdir($dirs[$dirselect]);
# traverse the directory tree
find(sub {
	return if (!-f $_); # ignore directories and other non regular files
	# register the relative path in the hash
	# see File::Find documentation for $File::Find::dir and $_
	# here name of current directory is $File::Find::dir
	# and name of directory entry is $_
	$b_paths{"$File::Find::dir/$_"} = undef;
}, '.');
chdir('..');

# switch to the first @dirs entry
$dirselect = 0;
chdir($dirs[$dirselect]);
# For each path in the directory check, 
# whether it is also present in the 'b' directory.
# This is done with a fast lookup in the hash %b_paths (set lookup)
# Print the result along with the path
# traverse the directory tree
find(sub {
	return if (!-f $_); # ignore directories and other non regular files
	my $p = "$File::Find::dir/$_";
	my $exists = exists $b_paths{$p} ? '' : ' not';
	$p =~ s{^\.}{$dirs[$dirselect]}xms; # change the leading '.' for the base directory name
	print "Path $p does$exists exist in the '$dirs[1-$dirselect]' directory\n";
}, '.');
chdir('..');