This is sort of quick and dirty, but it's a start:

my %out;
while (<DATA>) {
    if ( /##head##/ ) {
         if ( %out ) {
             print join ',', $out{servername}, @{ $out{serverip} };
             print "\n";
         }
         %out = ();
     }
    $out{servername} = $1 if /^servername\s+(.*)/;
    push @{$out{serverip}}, $1 if /^serverip:\s+(.*)/;
}
__DATA__
##head##
servername abc
xxx
xxx
serverip: 123
xxx
##head##
--------
##head##
servername def
xxx
xxx
serverip: 456
serverip: 445566
xxx
##head##
##head##
servername ghi
xxx
xxx
serverip: 789
xxx
##head##
[download]

Outputs:

abc,123
def,456,445566
ghi,789
[download]