my %hash = undef;

##</code><code>##

my %hash = ( key => "value", key2 => "value2" );   # OK
my %hash = ( "key", "value", "key2", "value2" );   # OK (same)
my %hash = ( "key", "value", "key2" );             # WRONG - gives the error you describe
my %hash = ( undef );                              # WRONG - equivalent to what you were doing!

##</code><code>##

undef %hash;
%hash = ();         # empty list = empty hash