The only one that is syntactically valid is the second one, however, I don't believe that it is doing what you are expecting. Since the parens denote precedence, what Perl reads it as is:

my %sham =  (
                1 => "j",
                2 => "b",
                1 => "p",
                2 => "b",
        );
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When assigning to hashes, later insertions with the same key end up replacing any earlier entries with the same key, leaving you with:

my %sham =  (
                1 => "p",
                2 => "b",
        );
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This can be shown with the use of Data::Dumper or the like. I think what you are intending is:

my @sham =  (               # note the sigil
               {
                1 => "j",
                2 => "b",
               },
               {
                1 => "p",
                2 => "b",
               },
        );
[download]

Data::Dumper (and any similar module) is a useful tool for visualizing nested data structures.