It can be loaded either way. It's totally up to you how you store the info in the hash.

Anyway, here's code that should accomplish your goal.

use strict;
use warnings;

my $fn_assn = ...;
my $fn_pssn = ...;

my @assns;
{
   open(my $fh_assn,  '<', $fn_assn)
      or die("Unable to open assn file \"$fn_assn\": $!\n");

   chomp(@assns = <$fh_assn>);
}

my @pssns;
{
   open(my $fh_pssn,  '<', $fn_pssn)
      or die("Unable to open pssn file \"$fn_pssn\": $!\n");

   chomp(@pssns = <$fh_pssn>);
}

{
   my %pssns = map { $_ => 1 } @pssns;
   my @unique_assns = grep { not exists $pssns{$_} } @assns;

   print("The following ASSNs have no corresponding PSSNs:\n");
   print("$_\n") foreach @unique_assns;
}

print("\n");

{
   my %assns = map { $_ => 1 } @assns;
   my @unique_pssns = grep { not exists $assns{$_} } @pssns;

   print("The following PSSNs have no corresponding ASSNs:\n");
   print("$_\n") foreach @unique_pssns;
}
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