Opposition to Python type hints

That $a and $b are predefined package globals in Perl 5. They should be used for sort blocks:

my @sorted = sort { $a <=> $b } @unsorted;

Try your example with $x and $y and you'll get:

~> perl x.pl
Global symbol "$y" requires explicit package name at x.pl line 6.
Global symbol "$y" requires explicit package name at x.pl line 6.
Execution of x.pl aborted due to compilation errors.

Yes, that's surprising behavior and it's one of the reasons for Perl 6 being a breaking change. Of course, $a and $b are not exactly good variable names anyway :)