At the expense of one more character you could use:

c:\test>perl -wle"sub TODO{ die 'Unimplemented' } 0 or TODO;"
Unimplemented at -e line 1.
[download]

with the same benefits. Or perhaps one of these:

c:\test>perl -wle"sub _'_'_{ die 'Unimplemented' } 0 or _'_'_;"
Unimplemented at -e line 1.

c:\test>perl -wle"sub ___{ die 'Unimplemented' } 0 or ___;"
Unimplemented at -e line 1.
[download]

I quite like that last one. And it has the added benefit of not requiring the use of a dog slow "real parser" to not confuse my syntax highlighter!

But perhaps it is more about the changes that allowed yadayada to be implemented, than yadayada itself.