%hoh = {
    # ...
};

##</code><code>##

$ perl -MData::Dumper -e'%hoh = { foo => 1 }; print Dumper \%hoh'
$VAR1 = {
          'HASH(0x812f180)' => undef
        };

##</code><code>##

$ perl -we'%hoh = { foo => 1 };'
Reference found where even-sized list expected at -e line 1.

##</code><code>##

%hoh = (
    # ...
);