# Snippet 3 - your answer?
use strict;
use warnings;
use Data::Dumper;

my @primary = qw(foo-Alice bar-Bob);
my @secondary = qw(p0-home p1-work p2-mobile);
my @tertiary = qw(itemA-555abcd itemB-555zxyw);

my %phonebook = ();

for my $primary_element (@primary) {
    for my $secondary_element (@secondary) {
            # Similar to method 1 from snippet 2a:
            # a reference directly to @tertiary
            $phonebook{$primary_element}->{$secondary_element} = \@tertiary;

            # -- or -- 
            
            # Similar to method 2 from snippet 2a:
            # a new anonymous array reference, containing the elements from @tertiary
            $phonebook{$primary_element}->{$secondary_element} = [ @tertiary ];
    }
}
print Dumper \%phonebook;