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", "subject4": "value"}], "subject5": "value", "subject6": "value", "subject7": "value"}, 
"2": {"subject1": "value", "subject2": [{"subject3": "value", "subject4": "value"}], "subject5": "value", "subject6": "value", "subject7": "value"}, 
"3": {"subject1": "value", "subject2": [{"subject3": "value", "subject4": "value"}], "subject5": "value", "subject6": "value", "subject7": "value"}
}
EOD

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