gistfile1.txt

#!/usr/bin/perl6-m
# -*-Perl-*-

use v6;

my @files = <file1.txt file2.txt file3.txt>;

my @all_counts = await do for @files -> $filename {
    start {
        my %word_counts;
        for slurp($filename).words {
            %word_counts{$_}++;
        }
        %word_counts;
    }
}

# Bring them together into a single result.
my %totals;

for @all_counts {
    %totals{.key} += .value;
}

say %totals.elems;