For what little it is really worth, you do not need the e modifier on this version ( as opposed to chromatic's suggestion ).

Both parts of the regex get double-quotish expansion, which will cause $hash{$1} to be expanded correctly.

The original problem was using the \1 as the backreference. That has been depricated since at least perl 5.004 ( maybe earlier, I cannot remember precisely ).

In fact, if you try to use the \1 on the command line and use perl -w, it will complain quite loudly. The moral here, of course, is make sure you use -w when developing code.

You also used the single quotes ('') around the variable, which would have prevented the \1 from being expanded even if \1 wasn't deprecated. Remember, single quotes mean literally and double quotes will do... well, double-quotish expansion.

Mik