The following code prints all the "leaves" of a complex data structure but you can add selection criteria operating on the keys as required.

use strict; 
use warnings;
use JSON;

my %dispatch = 
    (''        => sub { print "$_[0]\n"; },
     'HASH'    => sub { for my $key (keys %{$_[0]}) {
                            search_and_replace_in_hash( $_[0]->{$key} 
+);
                        } },
     'ARRAY'   => sub { search_and_replace_in_hash( $_ ) for @{$_[0]};
+ },
     'default' => sub { my ( $element, $op ) = @_;
                        print STDERR ref($element).": unknown type of 
+reference\n" },
    );
sub search_and_replace_in_hash {
  &{ $dispatch{ ref($_[0]) } // $dispatch{'default'} }(@_);
}

my $data = <<EOD;
{
"1": {"subject1": "value", "subject2": [{"subject3": "value", "subject
+4": "value"}], "subject5": "value", "subject6": "value", "subject7": 
+"value"}, 
"2": {"subject1": "value", "subject2": [{"subject3": "value", "subject
+4": "value"}], "subject5": "value", "subject6": "value", "subject7": 
+"value"}, 
"3": {"subject1": "value", "subject2": [{"subject3": "value", "subject
+4": "value"}], "subject5": "value", "subject6": "value", "subject7": 
+"value"}
}
EOD

$data = JSON->new()->decode( $data );
search_and_replace_in_hash( $data );
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