I believe this was not working until perl 5.0.

As per the docs:

Variables may now be interpolated literally into a pattern by prefixing them with \Q, which works just like \U, but backwhacks non-alphanumerics instead. There is also a corresponding quotemeta function.

On a side note though you might not want to get in the habit of using $a or $b as variable names as sooner or later you might want to sort something.

-enlil