Or think of these two text files as database tables and you can use the power of SQL and DBI to solve your assignment:

use Modern::Perl qw /2014/;
use DBI;
use Data::Dump qw /dump/;

my $dbh = DBI->connect("dbi:CSV:f_dir=.");
$dbh->{csv_allow_whitespace} = 1;

my $sql =
'SELECT names.firstname, names.name, names.ID, scores.ID, scores.score
+ FROM names FULL OUTER JOIN scores ON names.ID = scores.ID';

my $sth = $dbh->prepare($sql);
$sth->execute();
while ( my $row = $sth->fetchrow_hashref ) {
    say dump($row);
}
$sth->finish();
[download]

Output:

{
  "names.firstname" => "John",
  "names.ID"        => 34267,
  "names.name"      => "Kowalski",
  "scores.ID"       => undef,
  "scores.score"    => undef,
}
{
  "names.firstname" => "Eve",
  "names.ID"        => 11232,
  "names.name"      => "Nowak",
  "scores.ID"       => 11232,
  "scores.score"    => "5.0",
}
{
  "names.firstname" => "Theodor",
  "names.ID"        => 3423,
  "names.name"      => "McNack",
  "scores.ID"       => 3423,
  "scores.score"    => 3.5,
}
{
  "names.firstname" => undef,
  "names.ID"        => undef,
  "names.name"      => undef,
  "scores.ID"       => 55342,
  "scores.score"    => "2.0",
}
[download]

The only change I had to make to these text files was to add a first line with the field names.

The SQL for the exam results is SELECT names.firstname, names.name, scores.score FROM names JOIN scores ON names.ID = scores.ID

The list of absent students can be found with this SQL: SELECT names.firstname, names.name FROM names LEFT OUTER JOIN scores ON names.ID = scores.ID WHERE scores.score IS NULL and the unidentified results can be found with SELECT scores.ID, scores.score FROM names RIGHT OUTER JOIN scores ON names.ID = scores.ID WHERE names.name IS NULL