If the plan is to run a script at some interval (e.g. via your crontab), and you just want to monitor a single directory (not recursively watching all existing subdirectories), something like this would be pretty quick:

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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If you need to recursively check subdirs within the target path, you'll want to check out one of the various "File::Find" modules -- or just do something like:

my @new_list = split /\x0/, `find $chkpath -type d -print0`;
[download]

(which will also give you subdir names that begin with "."). -- Update: I'm sure you can get File::Glob to list dot-initial file names, too, but "that is left as an exercise..." ;)

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.