my @arr = qw/1 2 3 4 5 6 7 8/;
push @arr, undef;
push @arr, 9;


my ($j,$i)=(0,0);
my ($x,$y) = ([grep { ++$i % 2} @arr] , [grep {$j++ % 2} @arr]);
$i=0;
for(@{$x}){
  say $_ ." => " . $y->[$i++] if($_);
}