Your statement:

my %hash = undef;
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is incorrect. In this context, since %hash is more like an array than a scalar, 'undef' is actually a value you're trying to set to %hash. This "list-ish" behavior lets you do stuff like this:

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 - equivalen
+t to what you were doing!
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If you're wanting to "un-set" %hash, try either:

undef %hash;
%hash = ();         # empty list = empty hash
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But in your code above, this is unnecessary. Simply declaring my %hash; is sufficient, since in Perl this would initialize your hash to be empty.