I also ran parallel using MCE; 7 workers, each processing a range of characters.

#!/usr/bin/env perl
# https://perlmonks.org/?node_id=11148669

use warnings;
use strict;
use Judy::HS qw/ Set Get Free /;
use Sort::Packed 'sort_packed';
use MCE;

my $DATA_TEMPLATE    = 'nZ10';
my $DATA_SIZE        = 12;
my $COUNT_SIZE_BYTES = 2;
my $COUNT_SIZE_BITS  = 16;
my $COUNT_MAX        = int( 2 ** $COUNT_SIZE_BITS - 1 );

@ARGV or die "usage: $0 file...\n";
my @llil_files = @ARGV;

for (@llil_files) {
    die "Cannot open '$_'" unless -r "$_";
}

# MCE gather and parallel routines.
my $DATA = '';

sub gather_routine {
    $DATA .= $_[0];
}

sub parallel_routine {
    my $char_range = $_;
    my ( $data, $current, $judy ) = ( '', 0 );

    for my $fname (@llil_files) {
        open( my $fh, '<', $fname ) or die $!;
        while ( <$fh> ) {
            if (/^[${char_range}]/) {
                chomp;
                my ( $word, $count ) = split /\t/;

                ( undef, my $val ) = Get( $judy, $word );
                if ( defined $val ) {
                    vec( $data, $val * $DATA_SIZE / $COUNT_SIZE_BYTES,
                        $COUNT_SIZE_BITS ) -= $count
                }
                else {
                    $data .= pack $DATA_TEMPLATE, $COUNT_MAX - $count,
+ $word;
                    Set( $judy, $word, $current );
                    $current ++
                }
            }
        }
        close $fh;
    }

    Free( $judy );
    MCE->gather( $data );
}

# Run parallel using MCE.
warn "my_test start\n";
my $tstart1 = time;

MCE->new(
    input_data  => ['a-d','e-h','i-l','m-p','q-t','u-x','y-z'],
    max_workers => 7,
    chunk_size  => 1,
    posix_exit  => 1,
    gather      => \&gather_routine,
    user_func   => \&parallel_routine,
    use_threads => 0,
)->run(1);

my $tend1 = time;
warn "get_properties : ", $tend1 - $tstart1, " secs\n";
my $tstart2 = time;

sort_packed "C$DATA_SIZE", $DATA;
$| = 0;  # enable output buffering

while ( $DATA ) {
    my ( $count, $word ) = unpack $DATA_TEMPLATE, substr $DATA, 0, $DA
+TA_SIZE, '';
    printf "%s\t%d\n", $word, $COUNT_MAX - $count
}

my $tend2 = time;
warn "sort + output  : ", $tend2 - $tstart2, " secs\n";
warn "total          : ", $tend2 - $tstart1, " secs\n";

__END__

$ time perl mce_judyhs.pl big1.txt big2.txt big3.txt >out3.txt
my_test start
get_properties : 5 secs
sort + output  : 5 secs
total          : 10 secs

real  0m9.794s
user  0m35.719s
sys   0m0.257s
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