If you have "use warnings;" in your code (or run with "perl -w"), you should get the warning:

Reference found where even-sized list expected
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If you don't have "use warnings;", then put it in before asking a question. The warnings are there to help you, not annoy you.

UPDATE: And, if the warning is a little obscure, which in this case it is if you haven't seen it before, you can also "use diagnostics;". For example, perl -Mdiagnostics -w pm602312.pl gives:

Reference found where even-sized list expected at pm602312.pl line 2
 (W misc) You gave a single reference where Perl was expecting a list
 with an even number of elements (for assignment to a hash). This usua
+lly
 means that you used the anon hash constructor when you meant to use
 parens. In any case, a hash requires key/value pairs.
    
        %hash = { one => 1, two => 2, };        # WRONG
        %hash = [ qw/ an anon array / ];        # WRONG
        %hash = ( one => 1, two => 2, );        # right
        %hash = qw( one 1 two 2 );                      # also fine
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