#!perl -w

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

my @a = (1, 2, 3);
my @b = (4, 5, 6);

my @s = foo(@a, @b);

print Dumper(\@a, \@b, \@s);

sub foo {
  my ($t, $u) = (@_);  # $t = 1 ; $u = 2
  $u = undef;          # $a[1] == 2
  my $i = shift;       # $i = 1
  my $j = shift;       # $j = 2
  $_[0] = undef;       # $a[2] = undef
  my $k = shift;       # $k = undef
  $_[0] = undef;       # $b[0] = undef
  my $x = $_[0];       # $x = undef
  $k = $i + $j;        # $k = 1 + 2 ; $a[2] == undef
  $x = $i - $j;        # $x = 1 - 2 ; $b[0] == undef
  my $y = shift;       # $y = undef
  my @z = @_;          # @z = [5, 6]
  $z[0] = undef;       # @z = [undef, 6] ; $b[1] == 5
  return $t,$u,$i,$j,$k,$x,$y,@z;
}

__END__

$VAR1 = [
          1,
          2,
          undef
        ];
$VAR2 = [
          undef,
          5,
          6
        ];
$VAR3 = [
          1,
          undef,
          1,
          2,
          3,
          -1,
          undef,
          undef,
          6
        ];