You want something like

use Cpanel::JSON::XS qw( decode_json encode_json );
use File::Slurper    qw( read_binary );

my $watchers_data = decode_json( read_binary( $ARGV[0] ) );

my %watchers_lkup =
   map { $_->{ externalId } => $_->{ watchers } }
      @$watchers_data;

my $data = decode_json( read_binary( $ARGV[1] ) );

$_->{ watchers } = $watchers_lkup{ $_->{ externalId } }
   for @$data;

print encode_json( $data );
[download]

Using jq:

jq -n '
   (
      input |
      map( { key: .externalId, value: .watchers } ) |
      from_entries
   ) as $watchers_lkup |
   inputs |
   map( .watchers = $watchers_lkup[ .externalId ] )
' watchers.json file.json
[download]

Demo on jqplay.