If you are going to use strict; use warnings; (and it is a really really good idea), then you really really need to declare your variables: ;)

use strict;     # a good idea
use warnings;   # and another

my $input = 'myinput.txt';

open(my $in, "<", $input) or die "Could not open $input because $!\n";

my @unsorted = <$in>;
chomp (@unsorted); # get rid of end-of-record markers
close ($in);

my @sorted = sort @unsorted;
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