use 5.028;
use strict;
use warnings;

my ($fh, @athena, %melete);
open $fh, "<", "$ENV{HOME}/tmp/athena_msgids";
push @athena, $_ while <$fh>;
close $fh;
open $fh, "<", "$ENV{HOME}/tmp/melete_msgids";
$melete{$_} = undef while <$fh>;
close $fh;

for (@athena) {
    exists $melete{$_} or say "on athena but not melete: $_";
}