The first step: Fix the input. In your code sample, you created three arrays, each of them with just one member: an array reference.

Square brackets introduce array references. Use round parentheses () for lists.

@array1 = ('a','b','c');
@array2 = ('a','b');
@array3 = ('a','b','c','d');
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How do you want to group the results?

#! /usr/bin/perl
use warnings;
use strict;

use Syntax::Construct qw{ // };

my @array1 = ('a','b','c');
my @array2 = ('a','b');
my @array3 = ('a','b','c','d');

while (@array1, @array2, @array3) {
    print join ', ', map shift @$_ // 'NULL', \@array1, \@array2, \@ar
+ray3;
    print "\n";
}
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