Anonymous hashes are references to hashes that aren't named:

my %normal_hash = ( a => 1, b => 2 );
print $normal_hash{a};

my $hash_ref = \%normal_hash;
print $hash_ref->{a};

my $anon_hash = { a => 1, b => 2 }; # note the curly braces

print $anon_hash->{a};
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See also the perldata and perlref documentation.