Your statement: 
my %hash = undef;

[download]
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!

[download]
If you're wanting to "un-set" %hash, try either: 
undef %hash;
%hash = ();         # empty list = empty hash

[download]
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.

In reply to Re: Odd number of elements in hash assignment warning. Use of uninitialized value warning by Fastolfe
in thread Odd number of elements in hash assignment warning. Use of uninitialized value warning by princepawn

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.