sub foo {
    my $self = shift;

    # Is there a variable at all ( even undef )
    my $anything = @_ ? shift : return undef;

    # Is there a defined argument
    my $def = defined $_[0] ? shift : return undef;

    # Is there a true argument
    my $true = $_[0] ? shift : return undef;

    # Is there an argument which conforms to a regex?
    my $re_ok = $_[0] =~ /^\w+$/ ? shift : return undef;

    # If the argument one of a limited number of values
    my $option = $_[0] =~ /^(?:this|that|foo|bar)$/ ? shift : return undef;

    # Is it a reference
    my $ref = ref $_[0] ? shift : return undef;