Your code does not do what you claim. No arrays get merged when I run it. Please try to give more details: should the arrays be merged only if the last member is the same, or if any member is the same (i.e., should the script merge qw/a b key c/ with qw/x key y z/)? What would you do if more than one array from the second list could be merged into an array from the first one (i.e. both qw/car dog age/ and qw/train cat age/ in the second list)?

I've been staring at the screen for too long..

It's funny you say it doesn't merge, I've just copied the code from the post and used Data::Dumper to dump $arr at the end and I can see it merged as it should be..

No, the initial position of the member doesn't matter. If we look at an arbitrary first row of two arrefs:

$a->[0] = [qw/f0 f1 f2 f3/];
$b->[0] = [qw/e0 f2 e1 f0 e2];
[download]

If the value of the field in $b->[0] matches one already present in $a->[0] then all values in the matching field in the $b->1..$ arrays should be set in the already matched field. So the final array would be like:

$m = [
[qw/f0 f1 f2 f3 e0 e1 e2],
[qw/a0 a1 a2 a3 "" "" ""/],# ..and so on all of $a
[qw/b3 "" b1 "" b0 b2 b4/],# ..and so on for $b->[1..$]
[download]

a0,1,2.. being the first,second,third element of a $a->[x] array and likewise for the b0,1,2 of the $b->[x] array.

Does this make it any clearer?

Oh, wait, now I understand. The first member of the array is the header! I can see now what you mean by "merge", I imagined something different, which did not happen. I apologize for offending your code. Here is how I would do it. How sexy it is depends on the beholder's perspective:

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

use Data::Dumper;

my $arr = [[qw/name pos   loc age/],
           [qw/ike  boss  12  44/],
           [qw/mat  slave 22  21/],
           [qw/jill sec   15  32/],];

my $add = [[qw/car dog  age/],
           [qw/a1  grr  3/],
           [qw/s2  miew 7/],];

my $i = 0;
my %headers;
for (@{ $arr->[0] }, @{ $add->[0] }) {
    $headers{$_} = $i unless exists $headers{$_};
    $i++;
}

my $result = [ [ sort { $headers{$a} <=> $headers{$b} } keys %headers 
+] ];

my $new_columns = keys(%headers) - @{ $arr->[0] };
for (1 .. $#{$arr}) {
    push @$result, [ @{ $arr->[$_] }, (q()) x $new_columns ];
}

my @rearranged = map $headers{$_}, @{ $add->[0] };
for my $row (1 .. $#{$add}) {
    my @new = (q()) x keys %headers;
    my $i = 0;
    $new[$_] = $add->[$row][$i++] for @rearranged;
    push $result, [@new];
}
print Dumper $result;
[download]

لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ