It's exactly the reverse. An anonymous hash needs to be created every time you assign it, because the only thing you can store is a reference to it. The difference shows in the two programs:

#!perl -w
use strict;
use Data::Dumper;
my @hashes;
for (1..2) {
    push @hashes, { foo => 'bar' };
};
$hashes[0]->{baz} = 1;
print Dumper $hashes[1]; # { foo => 'bar' }
[download]

Your expectation would change this to the following:

<c>
#!perl -w
use strict;
use Data::Dumper;
my @hashes;
my $constant_hash = { foo => 'bar' };
for (1..2) {
    push @hashes, $constant_hash;
};
$hashes[0]->{baz} = 1;
print Dumper $hashes[1]; # { foo => 'bar', baz => 1 }
[download]