If i guessed your specs right you are after an intersection.

Hence something like this using Set::Scalar and Path::Tiny might be a way to go:

#!/usr/bin/env perl

use strict;
use warnings;
use Set::Scalar;
use Path::Tiny;
use Data::Dump;
use feature qw (say);

say path("file01.txt")->slurp_utf8;
say path("file02.txt")->slurp_utf8;

my @s = path("file01.txt")->lines_utf8( { chomp => 1 } );
my @t = path("file02.txt")->lines_utf8( { chomp => 1 } );

dd \@s;
dd \@t;

my $s = Set::Scalar->new(@s);
my $t = Set::Scalar->new(@t);
my $i = $s->intersection($t);

say $i;

path("out.txt")->spew_utf8( map { "$_\n" } @$i );

print path("out.txt")->slurp_utf8;

__END__

karls-mac-mini:sabas karl$ ./sabas.pl
foo
bar
donald

nose
cuke
donald

["foo", "bar", "donald"]
["nose", "cuke", "donald"]
(donald)
donald
[download]

OK, admittedly very simplified. And 300K isn't really large IMHO. Threads From Hell #2: How To Search A Very Huge File [SOLVED] might probably also be of interest. If you want to get rid of this @ARGV stuff see Re: GetOpt Organization. Some might say that i repeat myself ;-)

Best regards, Karl