Trying to puzzle out what the heck is meant by %{{ }} = LIST, I did

$ perl -MO=Deparse -e'print join( "|", %{{ }} = (1)x100 );'
print join('|', %{{};} = (1) x 100);
-e syntax OK
$ perl -e'print join( "|", %{{};} = (1)x100 );'
syntax error at -e line 1, near "%{{};"
Unmatched right curly bracket at -e line 1, at end of line
Execution of -e aborted due to compilation errors.
$
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That looks like a borderline syntax error, since perl parses the code to something just as weird, but differently. Or maybe it's an O::Deparse problem.

The behavior you noted goes away if all the hash key slots are different:

$ perl -we'print join( "|", %{{ }} = 1..100 );'
1|2|3|4|5|6|7|8|9|10|11|12|13|14|15|16|17|18|19|20|21|22|23|24|25|26|2
+7|28|29|30|31|32|33|34|35|36|37|38|39|40|41|42|43|44|45|46|47|48|49|5
+0|51|52|53|54|55|56|57|58|59|60|61|62|63|64|65|66|67|68|69|70|71|72|7
+3|74|75|76|77|78|79|80|81|82|83|84|85|86|87|88|89|90|91|92|93|94|95|9
+6|97|98|99|100$ 
$ perl -we'print join( "|", %{{ }} = (1,1,2,1,3,1,4,1,5,1) );'
1|1|2|1|3|1|4|1|5|1$
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Warnings are on, so the weirdness would show noisily.