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foreach my $hash (@a){
    foreach my $key (keys %{$hash}){
...
    }

}

use strict;
use warnings;
use diagnostics;

$a[$j]{$key} 
and 
$a[$j]->{$key}
both will work

for my $key (keys %{$a[0]})
according to your above presented logic

before calling the second for loop just try
use Data::Dumper;
print Dumper \@a;