UCD.pm: Add check for parameter type

[khwilliamson]

This needs to be a scalar reference; I got burned with it not doing the right thing when it wasn't.

• I'm unsure if this set of changes requires a perldelta entry

[tonycoz]

This could use a test.

• I'm unsure if this set of changes requires a perldelta entry, and it is included.

I don't see the perldelta entry.

[jkeenan]

This could use a test.

Provided one at khwilliamson#1

[tonycoz]

The test made obvious to me:

tony@venus:.../git/perl6$ ./perl -Ilib -MUnicode::UCD=num -e 'my $x; num("123", \$x)'
tony@venus:.../git/perl6$ ./perl -Ilib -MUnicode::UCD=num -e 'my $x=\1; num("123", \$x)'
Unicode::UCD::num: second parameter must be a scalar reference at -e line 1.

In both cases the parameter is a reference to $x, and $$x can be assigned to.

Another case would be:

tony@venus:.../git/perl6$ ./perl -Ilib -MUnicode::UCD=num -E 'my $x; num("123", bless \$x);'
Unicode::UCD::num: second parameter must be a scalar reference at -e line 1.

But I think that's more a case of "Doctor, it hurts when I do (something dumb)"

Alas builtin::reftype() also returns REF for the second case.

I'm not fond of this ref() behaviour (ref() -> REF describes the contents of the reference container, not the referenced container itself)

[jkeenan]

@tonycoz, what is your current opinion on this pull request? Can it be merged?

(Your May 19 comments suggest that in the course of reviewing this ticket, you came across larger problems with perl that are beyond the scope of this ticket. Is that impression correct?)