#!/usr/bin/perl -wT
use strict;

my @a = qw(1 2 3 4 5 6);
my @b = qw(a b c d e f g h i);

my $max = $#a > $#b ? $#a : $#b;  # find the max index of the biggest array

for my $i (0..$max) {             # loop through using the same index variable
  print "$a[$i]\n" if defined $a[$i];
  print "$b[$i]\n" if defined $b[$i];
}
=OUTPUT
1
a
2
b
3
c
4
d
5
e
6
f
g
h
i