"HASH" eq ref($_)
    /HASH/
    /(^|=)HASH\b/
    UNIVERSAL::isa( $_, "HASH" )

##</code><code>##

    use Hash::FindKeys qw( FindHashKeys );
    # FindHashKeys finds keys of a hash where the key and
    # corresponding value meet certain criteria.  Note that
    # ref($_[0]) must be "HASH" or it dies!
[...]
    sub someMethod {
        my( $self )= shift @_;
        my @opts= FindHashKeys( $self,
            sub { /^-/ }, sub { $_ ne "" } );
        # Oops, that dies!
    }