The error only comes about because there are 17 items altogether in your hash assignment. If you had one more URL, for example, you'd be stuck with a hard-to-find error. You're lucky.

If you mean to store a list in a hash, you'll have to create a reference, either by creating an array and storing a reference to it as the value in the hash, or making an anonymous list right there. Let's take the latter approach:

my %Config = (
    PPingPath => '/home/dean/pping-v1.37/pping',
    PPingOptions => '-t 15 -q |',
    TraceRTPath => '/usr/sbin/traceroute',
    TRHops => '-m 3 | ',
    ExternalTarget => [
        'www.microsoft.com -p 80', 'www.netscape.com -p 80',
        'www.silverhand.co.uk -p 80', 'www.demon.net -p 80', 'www.whit
+ehouse.org -p 80'],
    Hosts => [
        'www.netscape.come -p 80', 'www.microsoft.com -p 80']
);
[download]

See perlref for more information. (And remember that a hash can only store scalar values -- as a reference can be stored in a scalar, you can build more complex data structures by references. perldsc is the next place to look.)