foreach my $hash (@a){
    foreach my $key (keys %{$hash}){
        print "key >$key< value>",$hash->{$key},"<\n";
    }

}

##</code><code>##

use strict;
use warnings;
use diagnostics;

##</code><code>##

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

##</code><code>##

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

##</code><code>##

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