With Path::Tiny, you don't need File::Find.

#!/usr/bin/perl

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

use strict;
use warnings;
use Path::Tiny;

my $directory = 'some/test/';

path($directory)->visit
  (
  sub
    {
    my %u;
    -f and /\.txt$/ and $_->edit_lines
      (
      sub
        {
        $_ x= !$u{$_}++
        }
      )
    }
  );
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