my $current = sprintf("%02d%02d%02d",$std, $min, $sek);                                                 
my $past = $current - 10000;   
### Beide folgenden Varianten werfen den Fehler  use of unintialized value  ...                                                                         
#$past = sprintf("0%05d", $past) if($past != /\d{6}/);
#$past = "0$past" if($past !=~ m/\d{6}/);
if($past =~ m/\d{6}/g) {
        $past = $past;
}
else {
        $past = "0$past";
}

@array=qx{awk '/^$past/,/^$current/'logfile|grep '<pattern>'};

...

$past = sprintf("%06d",($current - 10000));

while ( $line = <LOG> ) {
  if ( $line =~ m/$startzeit/ .. $line =~ m/$endezeit/ ) {
    # tu was
  }
}

while ( <LOG> ) {
  if ( /$startzeit/../$endezeit/ ) {
     # tu was
  }
}