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use warnings;
use strict;
...

open my $fh, '>', 'data.json' or die $!;
print $fh $json;

{"second_hash":{"work":"office","family":"home"},"first_hash":{"Vegeta
+ble":"tomato","fruit":"banana"}}

use warnings;
use strict;
...
my $data = decode_json $json;

print Dumper $data->{$want};

$ perl fetch_hash.pl first_hash

...
          'work' => 'office',
          'family' => 'home'
        };