Question

I needed to build a variable depth hash data structure in perl. Eventually I found this piece of code:

#!/usr/bin/perl -w
use strict;

my %hash;
my $value = "foo";
my @cats = qw(a b c d);

my $p = \%hash;
foreach my $item (@cats) {
 $p->{$item} = {} unless exists($p->{$item});
 $p = $p->{$item};
}

My question is how and why it works. I thought I knew how perl worked. At no point in this code do I see \%hash value being reset and it appears that $p (which is a local variable) is reset on every loop. I could even see it with data dumper:
Running:

#!/usr/bin/perl
use warnings;
use strict;
use Data::Dumper;

my %hash;
my $value = "foo";
my @cats = qw(a b c d);

my $p = \%hash;
foreach my $item (@cats) {
 print "BEFORE:\n";
 print Dumper(\%hash);
 #print Dumper($p);
 $p->{$item} = {} unless exists($p->{$item});
 $p = $p->{$item};
 print "AFTER:\n";
 print Dumper(\%hash);
 #print Dumper($p);
}

And Then uncommenting the line with

#print Dumper($p)

CLEARLY shows $p being a new variable every time.

How does \%hash gets build if $p is reset every time?

Answer 1

$p isn’t being “reset” each time; it’s being set to the next lower level in the hash, the one just added. At this point it is just an empty hashref, {}, because it will be filled in on the next loop iteration.

I’m not sure how you are concluding that $p is a new variable each time, but this is incorrect.. it’s just pointing to a new location in the %hash structure. Since it was declared outside the loop, it doesn’t lose its value as the loop iterates.