sub isAnAllowedCharacter {
    my $testee = shift;

    ( $testee eq 'a' || ... )

    # or perhaps

    %valid_characters = map { $_ => 1 } ( 'a' .. 'd' );
    $valid_characters{ $testee };

    # or even perhaps
    my $validation_service = Remote::Validation::Service->new(...);
    $validation_service->isValid( $testee );
}