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

}

[download]
use 'Data::Dumper' module to print your data structure @a, you can visualize what you have created.
try using 
use strict;
use warnings;
use diagnostics;

[download]

show the code of routine 'create_hash_for_filename'
UPDATE 
$a[$j]{$key} 
and 
$a[$j]->{$key}
both will work

[download]
you are using @a for merging, but finally expecting output in @octopus_plid, how it is possible?.

UPDATE
nothing wrong in using like 
for my $key (keys %{$a[0]})
according to your above presented logic

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

[download]

run your script under perl -d
debugger will help you definitely. see 'man perldebug' for usage of debugger
Vivek
-- In accordance with the prarabdha of each, the One whose function it is to ordain makes each to act. What will not happen will never happen, whatever effort one may put forth. And what will happen will not fail to happen, however much one may seek to prevent it. This is certain. The part of wisdom therefore is to stay quiet.

In reply to Re: array of hashes question by targetsmart
in thread array of hashes question by jimmy78

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