#!/usr/bin/perl -w

use strict;

my @tag_a;
my @pos_i1;
my @pos_i2;
my @name;

my $datei;
my $tr_id;
my $zeile;
my $zeile_1;
my $p1;
my $p2;
my $id;
my $i;
my $itag;
my $ok;
my $max_i;
our $plus;
my $k;

...

open (IN,"<$datei") || die $!;
while(<IN>){
        ...
  $plus = 0;
  
  while($i <= $max_i - 1){
              ...
    if ($zeile eq $tag_a[$i]){
                     ...
      SWITCH: {
        $i == 0 && do { print "$name[$i]:           $_";
                        last SWITCH; };     
        $i == 1 && do { $plus = -1;
                        print "$i     Plus: $plus\n";
                        last SWITCH; };
=====>      $i == 2 && do { print "=====> $i          $plus\n";
                        $k = $i + $plus;
                        print "$k    $name[$k]:           $_"; 
                        last SWITCH; };
        $i == 3 && do { print "$name[$i]:           $_"; 
                        last SWITCH; };
      print "\$i is not in 0..3\n";
      } 
      $i = $i + 1;    
    }
    else {
      $i = $i + 1;
    } 
  } 
}
close IN;