open DAT, "ads.dat" or die "Error: cannot open data file: $!\n";

@imgdata=<DAT>;
@imgs = grep { s/^[^|]+\|([^|]+)\|[^|]+\|(?:internal|partner)$/$1/ } @imgdata;

$img  = $imgs[int(rand($#imgs+1))];
print "Location: $img\n\n";

##</code><code>##

s/^            # Beginning of string
   [^|]+       # Everything up to the next bar
   \|          # The bar
   (           # Capture to $1
     [^|]+     # Everything up to the next bar
   )           # End capture
   \|          # The bar
   [^|]+       # Everything up to the next bar
   \|          # The bar
   (?:         # Non-capturing parens
     internal  # the string "internal"
     |         # or
     partner   # the string "partner"
   )$          # End of string
   /$1/x       # Substitute $1 for string