my %is_legal = map { $_ => 1 } qw(
    y yes n no
);

my $answer;
{
    print "What is your answer? ";
    chomp( $answer = <> );
    $answer = lc $answer;
    last if $is_legal{ $answer };
    redo;
}

# Do something with $answer, which is now guaranteed to be 'y', 'yes', 'n', or 'no'