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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