pure-perl.pl

sub needs-change($o) { $o->name eq 'Seek' }
sub change($o) {
  my %props = %o{qw/name level x y/};
  my @perks = @$o->perks;
  push @perks, $perk_to_add;
  %props{perks} = \@perks;
  bless %props, ref($o);
}
my @updated = map { needs-change($_) ? change($_) : $_ } @objects;

with-lenses.pl

# Lens definition -- needs to be done one per key, not every time you have such an operation!
my $perks = immutable::lens->make(sub { $_[0]->{perks} }, sub { clone $_[0], perks => $_[1] });
# Actual code
sub with_name($name) { lens->grep({$_->name eq $name}); }
my @updated = with_name()->compose($perks)->modify(@objects, { [@$_[0], $new_perk] });