AAAAAAA¡aaaaaaa¡aaa
BBBB¡bbbb¡bbb
CCCCCC¡cccc¡ccc
DDDDDDDD¡ddddd¡dddd
EEEEE¡eeeeeeeeee¡eee
FFFFFFFF¡fffff¡ff
GGGGGG¡ggggggggggggg¡ggg
HHHHH¡hhhhh¡h

#!/usr/bin/perl
use warnings; use strict; use diagnostics;
use Fcntl qw(:DEFAULT :flock SEEK_SET SEEK_CUR);
sub systell { sysseek($_[0], 0, SEEK_CUR) }

sysopen FH, "datei.txt", O_RDWR or die "Kann Datei nicht öffnen: $!\n";
flock FH, LOCK_EX or die "Kann Datei nicht sperren: $!\n";
my $pos = 0;
my ($feld1, $feld2, $feld3) = "";
while (<FH>) {
    chomp;
    ($feld1, $feld2, $feld3) = split /\xA1/;
    if ($feld1 eq "EEEEE") {
        sysseek FH, $pos, SEEK_SET;  # möchte aktuelle Zeile überschreiben
        print FH "E-E-E¡eeeeeeeeee¡eee\n";  # geänderte Zeile
    };
    $pos = main::systell FH;  # ich möchte den Zeilenanfang treffen
}
close FH;

use strict;
use warnings;
use Fcntl qw/ SEEK_CUR O_RDONLY LOCK_EX /;

sub sys_verwenden {
   print '-' x 30, "\n";

   sysopen my $fh, "file.txt", O_RDONLY or die $!; 
   flock $fh, LOCK_EX or die $!; 

   sysread($fh, my $buf, 50);
   my $pos = sysseek($fh, 0, SEEK_CUR);
   print "Position: $pos\n";
   print "Buffer:\n";
   print $buf, "\n";
   close $fh;
}

sub vermischt {
   print '-' x 30, "\n";

   sysopen my $fh, "file.txt", O_RDONLY or die $!; 
   flock $fh, LOCK_EX or die $!; 

   while (my $line = <$fh>) {
      printf "%5s %s", sysseek($fh, 0, SEEK_CUR), $line;
   }   

   close $fh;
}

sub ohne_sys {
   print '-' x 30, "\n";

   sysopen my $fh, "file.txt", O_RDONLY or die $!; 
   flock $fh, LOCK_EX or die $!; 

   while (my $line = <$fh>) {
      printf "%5s %s", tell($fh), $line;
   }   

   close $fh;
}

sys_verwenden();
vermischt();
ohne_sys();

use strict;
use warnings;
use Fcntl qw(:DEFAULT :flock SEEK_SET SEEK_CUR);
use utf8;

sysopen my $fh, "datei.txt", O_RDWR or die "Kann Datei nicht öffnen: $!\n";
flock $fh, LOCK_EX or die "Kann Datei nicht sperren: $!\n";
binmode $fh, ':utf8';

my $pos = 0;

while (my $line = <$fh>) {
    chomp($line);
    my ($feld1, $feld2, $feld3) = split /\xA1/, $line;
    if ($feld1 eq "EEEEE") {
        seek($fh, $pos, SEEK_SET);
        print $fh "E-E-E¡$feld2¡$feld3\n";
    };  
    $pos = tell($fh);
}

close $fh;

sub sys_seek {
   sysopen my $fh, "file.txt", O_RDONLY or die $!; 
   flock $fh, LOCK_EX or die $!; 
   print "Position: ", sysseek($fh, 0, SEEK_CUR), "\n";
   <$fh>;
   print "Position: ", sysseek($fh, 0, SEEK_CUR), "\n";
}

foo();