#!perl
use 5.020;
use JSON::Tiny 'decode_json', 'encode_json';

sub read_json {
    my ($filename) = @_;
    open my $fh, '<', $filename
        or die "Couldn't read '$filename': $!";
    return decode_json( do { local $/; <$fh> } ); # sluurp
}

my $first_json_file = 'first.json';
my $second_json_file = 'second.json';
my $data_first = read_json( $first_json_file );
my $data_second = read_json( $second_json_file );

my %watchers;
for my $w ($data_first->@* ) {
    $watchers{ $w->{ externalId } } = $w->{ watchers };
}

for my $o ($data_second->@* ) {
    my $id = $o->{externalId};
    if( ! $watchers{ $id } ) {
        warn "Second item '$id' does not have watchers?!";
    } else {
        $o->{ watchers } = $watchers{ $id };
    }
};

say json_encode( $data_second );