use URI;

my %hostcount;

while (<DATA>)
{
    my $u = URI->new($_);
    $hostcount{$u->host}++;
}

foreach my $host (keys %hostcount)
{
    print "$host = $hostcount{$host}\n";
}

__DATA__
http://riverserver/dir1/dir2/index.html
http://riverserver/dir1/dir2/index.html
http://perlmonks.org/index.pl
http://webserver.company.serv/Adir/Bdir/thePage.cfm
http://riverserver/dir1/dir2/index.html
http://webserver.company.serv/Adir/Bdir/thePage.cfm