Just another example of where Perl automagically creates data structures as needed:

#!/usr/bin/perl

use strict;
use warnings;

my $href;            # not initialized

for my $key (keys %$href) {
    # ... do something
}

use Data::Dumper;
print Dumper $href;  # --> $VAR1 = {};
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which shows that $href has been autovivified to hold a reference to an (empty) hash, simply because you were using the scalar in a way (keys %$href) that would not have worked without such a data structure...