use strict;
use warnings;
use Data::Dumper;

# Data infered from posted output.
my @xfeld  = qw(0 0 y y 0 5 x 5 5 y);
my @xfix   = qw( UNKNOWN_x_0 UNKNOWN_x_1 UNKNOWN_x_2 3.640250E-4 );
my @yfix   = qw( UNKNOWN_y_0 UNKNOWN_y_1 UNKNOWN_y_2 3.640000E-4 );
my @fnfeld = (
    {fn => 'UNKNOWN_n_0',}, {fn => 'UNKNOWN_n_1',},
    {fn => 'UNKNOWN_n_2',}, {fn => 'UNKNOWN_n_3',},
    {fn => 'UNKNOWN_n_4',}, {fn => 'UNKNOWN_n_5',},
    {fn => 'UNKNOWN_n_6',}, {fn => 'UNKNOWN_n_7',},
    {fn => 'UNKNOWN_n_8',}, {fn => 'UNKNOWN_n_9',},
);
my @yfeld;
$#yfeld    = $#xfeld;

my %plotx;
my $ii = -1;
for (my $i = 0; $i <= $#yfeld; $i++) {
   $ii = $ii + 1;
   print "$i $ii ";
   if ($xfeld[$i] eq 'x') {
      push @{$plotx{$ii}}, @xfix;
      print "x\n";
   } elsif ($xfeld[$i] eq 'y') {
      push @{$plotx{$ii}}, @yfix;
      print "y\n";
   } else {
      push @{$plotx{$ii}}, $fnfeld[$xfeld[$i]]->{'fn'};
      # The Previous statement stores only one value.
      # The failing prints expects at least four.
      print "$xfeld[$i]\n";
   }
}

print "\n";
print "2 $plotx{2}[3]\n";
print "3 $plotx{3}[3]\n";
print "5 $plotx{5}[3]\n"; # Array element does not exist
print "6 $plotx{6}[3]\n";
print "7 $plotx{7}[3]\n"; # Array element does not exist
print "8 $plotx{8}[3]\n"; # Array element does not exist
print "9 $plotx{9}[3]\n";

print Dumper(\%plotx);

OUTPUT:
0 0 0
1 1 0
2 2 y
3 3 y
4 4 0
5 5 5
6 6 x
7 7 5
8 8 5
9 9 y

2 3.640000E-4
3 3.640000E-4
Use of uninitialized value in concatenation (.) or string at buchi2.pl line 41.
5
6 3.640250E-4
Use of uninitialized value in concatenation (.) or string at buchi2.pl line 43.
7
Use of uninitialized value in concatenation (.) or string at buchi2.pl line 44.
8
9 3.640000E-4
$VAR1 = {
          '1' => [
                   'UNKNOWN_n_0'
                 ],
          '8' => [
                   'UNKNOWN_n_5'
                 ],
          '6' => [
                   'UNKNOWN_x_0',
                   'UNKNOWN_x_1',
                   'UNKNOWN_x_2',
                   '3.640250E-4'
                 ],
          '3' => [
                   'UNKNOWN_y_0',
                   'UNKNOWN_y_1',
                   'UNKNOWN_y_2',
                   '3.640000E-4'
                 ],
          '2' => [
                   'UNKNOWN_y_0',
                   'UNKNOWN_y_1',
                   'UNKNOWN_y_2',
                   '3.640000E-4'
                 ],
          '9' => [
                   'UNKNOWN_y_0',
                   'UNKNOWN_y_1',
                   'UNKNOWN_y_2',
                   '3.640000E-4'
                 ],
          '7' => [
                   'UNKNOWN_n_5'
                 ],
          '5' => [
                   'UNKNOWN_n_5'
                 ],
          '0' => [
                   'UNKNOWN_n_0'
                 ],
          '4' => [
                   'UNKNOWN_n_0'
                 ]
        };