#!/usr/bin/perl

# http://perlmonks.org/?node_id=1193719

use strict;
use warnings;
use File::Find;
use File::Slurp qw( edit_file_lines );

my $directory = 'some/test/';

find sub { my %u; -f and /\.txt$/ and
  edit_file_lines sub { $_ x= !$u{$_}++ }, $_ }, $directory;