in reply to "Change, but don't touch" or naive use of perl -pi

Do it in two passes: 
my @real;
my $target = qr/(:Timeplaner):Projekt,/;
while (<>) {
  if (/$target/) {
    push @real, $ARGV;
    close ARGV;
  }
}
@ARGV = @real;
$^I = "";
while (<>) {
  s/$target/$1$1,/;
  print;
}

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-- Randal L. Schwartz, Perl hacker
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In Section Seekers of Perl Wisdom