Solution to http://blogs.perl.org/users/ovid/2014/12/a-small-puzzle-for-you.html

gistfile1.pl

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

use Test::More tests => 3;

{   my @list;

    sub init {
        my %elem;
        undef @elem{@_};
        @list = sort keys %elem;
    }


    sub rewrite {
        for my $i (0 .. $#list) {
            splice @_, $i, 0, undef unless defined $_[$i] and $_[$i] eq $list[$i];
        }
        return @_
    }
}


my @a = qw( b c f );
my @b = qw( a d );
my @c = qw( c d e );

init(@a, @b, @c);

is_deeply([rewrite(@a)], [ undef,    'b',    'c',  undef,  undef,    'f' ], 'a');
is_deeply([rewrite(@b)], [   'a',  undef,  undef,    'd',  undef,  undef ], 'b');
is_deeply([rewrite(@c)], [ undef,  undef,    'c',    'd',    'e',  undef ], 'c');