Fix Perl code which had broken the Windows build

Fix Perl code which had broken the Windows build

The previous change wanted to avoid modifying $_ in grep, but the code
just made the change in a local variable and then lost it.  Rewrite the
code using a separate map and grep, which is clearer anyway.

Author: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>