I used Inline::Files so that I could produce a single post with runnable Perl code.

You can do that without a module by open'ing a reference to a HEREDOC. That would work for multiple input "files" as in your mentioned solution.

use strict;
use warnings;

use Data::Dumper;

open my $inFH, q{<}, \ <<EOD or die qq{open: < HEREDOC: $!\n};
COA213345
COA213345
COA213445
DOB213345
EOA213345
EOD

my %lookup = map { chomp; $_ => 1 } <$inFH>;

close $inFH or die qq{close: < HEREDOC: $!\n};

print Data::Dumper->Dumpxs( [ \ %lookup ], [ qw{ *lookup } ] );
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The output.

%lookup = (
            'EOA213345' => 1,
            'COA213445' => 1,
            'COA213345' => 1,
            'DOB213345' => 1
          );
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For the purposes of the OP's request, note that using a hash removes the duplicate value in the input.