Your code will print out multiple Location headers, because you don't stop after printing the first one. Also, you don't need to call srand.

Here's some code that will do the same job (hopefully):

    open DAT, "ads.dat" or die "Can't open data file: $!";
    my @valid =
        grep $_->[3] =~ /^internal|partner$/,
        map [ split /\|/ ], <DAT>;
    close DAT or die "Can't close data file: $!";

    my $which = $valid[ rand @valid ];
    print "Location: ", $which->[2], "\n\n";
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