if ( ! ref($x) ) {
    # Not a reference
}
elsif ( UNIVERSAL::isa($x,'HASH') ) {
    # Reference to a hash
}
elsif ( UNIVERSAL::isa($x,'ARRAY') ) {
    # Reference to an array
}
elseif ( UNIVERSAL::isa($x,'SCALAR') || UNIVERSAL::isa($x,'REF') ) {
    # Reference to a scalar
}
elsif ( UNIVERSAL::isa($x,'CODE') ) {
    # Reference to a subroutine
}