my @alist = (
  [key1 => value1],
  [key2 => value2],
  [key3 => value3],
);

sub assq {
  my ($alistref, $key) = @_;
  for (@$alistref) {
     return $_ if $$_[0] eq $key;
     # eq doesn't mean exactly the same thing as in lisp,
     # but don't worry about that if you don't know lisp.
  }
}
sub rassq {
  my ($alistref, $value) = @_;
  for (@$alistref) {
     return $_ if $$_[1] eq $value;
     # eq doesn't mean exactly the same thing as in lisp,
     # but don't worry about that if you don't know lisp.
  }
}