Why does my value change when I am not resetting it? - perl

Question

I have the following example exhibiting the problem I'm struggling to resolve.
In the toy example, I have an array #actors with two levels.
I also have an array of hashes #people which I am using to 'look up' properties of the people in #actors.
The output of the program should be:
blue, blue cat, cat
red, red dog, dog
blue, blue cat, cat
red, red dog, dog
but instead I'm getting:
blue, cat cat, cat
red, dog dog, dog
blue, cat cat, cat
red, dog dog, dog
That is, it seems that in setting $favanim[$i][$j] I seem to be also overwriting the value of $favcols[$i][$j].
I suspect that for some reason the fact that #actors is a 2-dimensional array means that assignments via = are as references rather than as values, though I don't know why or how to stop it.
Please help!
The toy program is here: (I apologise if it can be simplified whilst still exhibiting the problem - it has taken me most of the afternoon to strip it down to this)
#!/usr/bin/perl -w
my #people = ();
$people[0]{'alternative full names for regexp'} = 'matthew smith|matt smith';
$people[1]{'alternative full names for regexp'} = 'david tennant|dave tennant';
$people[0]{'fav colour'} = 'red';
$people[1]{'fav colour'} = 'blue';
$people[0]{'fav animal'} = 'dog';
$people[1]{'fav animal'} = 'cat';
my #actors = ();
$actors[0][0] = 'David Tennant';
$actors[0][1] = 'Matt Smith';
$actors[1][0] = 'David Tennant';
$actors[1][1] = 'Matt Smith';
my #favcols = #actors;
my #favanim = #actors;
for ($i=0; $i<2; $i++) {
for ($j=0; $j<2; $j++) {
my #matching_people = grep{$actors[$i][$j] =~ m/^$_->{'alternative full names for regexp'}$/i} #people;
$favcols[$i][$j] = $matching_people[0]{'fav colour'};
$favanim[$i][$j] = $matching_people[0]{'fav animal'};
print "$matching_people[0]{'fav colour'}, $favcols[$i][$j] $matching_people[0]{'fav animal'}, $favanim[$i][$j]\n";
}
}

Answer 1

Try using
#favcols = map { [#$_] } #actors;
#favanim = map { [#$_] } #actors;
Deep copy vs shallow copy.

Answer 2

The problem is that you are initialising #favcols and #favanim by copying the contents of #people, which contains two array references
That sets $favcol[0] and $favanim[0] to a reference to the same array, [ 'David Tennant', 'Matt Smith' ], so when you modify $favcols[$i][$j] and then $favanim[$i][$j] you are overwriting the same array element
I don't see any reason to initialise your arrays at all, and if you declare them as just
my (#favcols, #favanim);
then you will find that your program does what you expect
By the way, you must always use strict, and use warnings is preferable to -w on the command line