It's working! I used choroba's advice and removed the whole if...else construct. Resultant loop looks like this: 
while (my $row= $csv->getline_hr($fh))
{
        $linecounter++;
        my $ip=$row->{'Source address'};
        my $url=$row->{'URL/Filename'};

        push @{ $ipURL{$ip} }, $url;

        if (!($linecounter % 50000))
        {
                print "Lines: $linecounter\n";
        }
}
        formatOutput(\%ipURL);
#       print Dumper \%ipURL;

[download]
The entire thing runs in about 45 seconds now: 
perl test-urlListbyIP.pl
Lines: 50000
Lines: 100000
Lines: 150000
Lines: 200000
Lines: 250000
Lines: 300000
Lines: 350000
Lines: 400000
Lines: 450000
Lines: 500000
Lines: 550000
Lines: 600000
Lines: 650000
Lines: 700000
Lines: 750000
Lines: 800000
Lines: 850000
Formatting Output...
List End:1316
Execution Time: 44.18 s

[download]

In reply to Re: Hash Search is VERY slow by rtjensen
in thread Hash Search is VERY slow by rtjensen

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.