That seems a little "brute-force", but other methods will depend on the OS: Win32, Mac/Unixy, or who-knows-what? I'll admit not knowing about anything but Win32 on that level.

Win32 provides monitoring API calls (FindFirstChange(), FindNextChangeNotification() and SHChangeNotifyRegister() all look like things to try.) You may have to code that part in XS, and other people would have more knowledge about specifics than I would (I remember using the FindXChangeNotification API calls in a C++ program a few years back.)

the script is running in debian 4.0

Linux::Inotify2.  Some sample code to get you started: Inotify2 findings.

Update: more (and specific) sample code:

#!/usr/bin/perl

use Linux::Inotify2;

my $inotify = Linux::Inotify2->new()
   or die "unable to create new inotify object: $!";

$inotify->watch(
    "/tmp",     # directory to watch
    IN_CREATE,  # flags
    sub {       # callback routine
        my $evt = shift;
        my $name = $evt->fullname;
        if (-d $name) {
            print "directory $name has been created\n";
        }
    }
);

1 while $inotify->poll;

__END__
$ stty -tostop   # just in case
$ ./753790.pl &
[1] 2590
$ mkdir /tmp/somedir
directory /tmp/somedir has been created
$
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use Croak qw/croak/;  # (update; don't forget this line, if making a m
+odule)

sub check_new_paths
{
    my ( $chkpath, $reflist ) =  @_;  # path to monitor, list of prev.
+ contents

    my %old_list;
    if ( -f $reflist ) {
        open( LST, "<", $reflist ) or croak( "Can't read $reflist: $!"
+ );
        %old_list = map { chomp; $_ => undef } <LST>;
        close LST;
    }
    if ( ! -d $chkpath ) {
        croak( "$chkpath is not a directory" );
    }
    my @new_list = grep { -d } <$chkpath/*>;
      # NB: file glob does not catch "$chkpath/.any-dot-initial-dirnam
+e"

    my @added_list;
    if ( keys %old_list ) {
        for my $p ( sort @new_list ) {
            push @added_list, $p unless exists( $old_list{$p} );
        }
    }
    else {
        warn sprintf( "Creating new list %s with %d items\n",
                      $reflist, scalar @new_list );
    }
    open( LST, ">", $reflist ) or croak( "Can't write $reflist: $!" );
    print LST "$_\n" for ( @new_list );
    close LST;

    return @added_list;
}
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my @new_list = split /\x0/, `find $chkpath -type d -print0`;
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But if you can get Linux::Inotify to work (shouldn't be too hard), it'll probably include better, more general tools that you'll want to use.

Another possibility...

#! /usr/local/bin/perl
use strict;
use warnings;
use File::Find ();
use List::Compare;
use Tie::Persistent;

my $root = shift or die "USAGE: $0 <directory>";
die "$root: not a directory" unless(-d $root);

# Get historic and current states
tie my @dirs, 'Tie::Persistent', './dirs.pd', 'rw';
my @newdirs = get_dirs($root);

# Check for changes
my $lc = List::Compare->new(\@dirs, \@newdirs);
print "Removed directories:\n\t" . join("\n\t", $lc->get_unique()) . "
+\n";
print "New directories:\n\t" . join("\n\t", $lc->get_complement()) . "
+\n";


# Update the persistent record
@dirs = @newdirs;

exit(0);




print "@dirs\n";

{
    my @dirs;

    sub wanted {
        my ($dev,$ino,$mode,$nlink,$uid,$gid);

        (($dev,$ino,$mode,$nlink,$uid,$gid) = lstat($_)) && -d _
        && push(@dirs, $File::Find::name);
    }

    sub get_dirs {
        my $root = shift;
        # Traverse desired filesystems
        File::Find::find({wanted => \&wanted}, $root);
        return(@dirs);
    }

}
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