{} creates an empty reference to an anonymous hash. Then,

for my $num (@a) {
   push @hashes,{}
}
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Reads: For each element in the array @a, create a reference to an empty hash and store it in the array @hashes

It could be written more compact as in:

push @hashes,{} for (@a);

or

$hashes[$_-1]={} for (@a);
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Hope this helps!

citromatik