# read
use Path::Tiny;
my @words = ( path($filename)->slurp ) =~ /^.+$/gm;

##</code><code>##

# optional read without Path::Tiny
my @words = do { local(@ARGV, $/) = $filename; <> =~ /^.+$/gm };

# or
my @words = do { local(@ARGV, $/) = $filename; split /\n/, <> };


##</code><code>##

# write
use Path::Tiny;
use List::Util qw( uniq );
path($filename)->spew(join "\n", uniq(@words), '');