From the docs:

ls ( DIR )
Get a directory listing of DIR, or the current directory.
In an array context, returns a list of lines returned from the server. In a scalar context, returns a reference to a list.

So your code should read

# Retrieve a recursive directory listing 
@ls = $ftp->ls();

foreach $file ( @ls ) 
{         
    print "$file<br>";
}
[download]