use 5.14.0;
my $muster = "Hier langer Vorname                             50#Hier der 50er Nachname                            #";
say "Vorname: ", substr($muster, 0, 50);
say "Nachname: ", substr($muster, 51, 50);

say "Zeile vorher:  ", $muster;
substr($muster, 0, 50) = "!" x 50;
substr($muster, 51, 50) = "*" x 50;
say "Zeile nachher: ", $muster;