Here's a Perl implementation:

  use strict; use warnings;
  use List::Util  qw(reduce);
  use File::Slurp qw(read_file);

  sub word_count {
  	reduce { $a->{$b}++; $a  } {},
  	map    { split(/\W+/)    }
  	map    { lc read_file $_ } @_;
  }