#!C:/Perl/bin/perl.exe

use DBI;
use warnings;
use mysql;
use HTML::Template;
use HTML;
use CGI;

my $cgi = CGI->new;
print $cgi->header();
my $kunden_id = $cgi->param('id_kunde'); 
gib_kunden_tabelle_aus($kunden_id);

my @data;
my $option = $cgi->param('option');
if ($option eq 'systeme') {
  @data = hole_systemids_uber_kunden_id($kunden_id);
}
if (@data) {
  # gib HTML für Systeme aus
  gib_system_tabelle_aus(@data);
  exit;
}
else {
  # gib HTML für Kunde aus
}


sub hole_systemids_uber_kunden_id {
    ($kunden_id) = @_;
    $query_kdsys = "select ID from tb_systeme where ID=?";
    $dbh         = DBI->connect( "DBI:mysql:projekt1:localhost", 'root', '' );
    $statement   = $dbh->prepare($query_kdsys);
    $statement->execute($kunden_id);
    my @row;
    while (@row = $statement->fetchrow_array() ) { # weil es mehrere System-IDs sein können
        push @data,\@row; # sichere eine Ergebniszeile ins Array
    }
    return @data; 
}

<td><a style='color:#000' href='?id_kunde=$id_kunde&option=systeme'>$kundenname</a></td>