How about a couple more tests?

use Data::Dumper;

my %h = 'a'..'f';
$_ .= '_mod' for %h;

print Dumper \%h;
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Here we try to modify every element in the 'hash', and the result show that only values can be changed, not the keys. But doesn't this make sense? Have you ever found yourself trying to change hash keys? You can't, you have to make a new key instead:

$h{a_mod} = delete $h{a};
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Now, let's take chromatic's advice and try this with an array:

my @h = 'a'..'f';
$_ .= '_mod' for @h;

%h = @h;
print Dumper \%h;
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Because at the time we changed the 'keys', they weren't yet keys. ;)