Fixed a UTF-8 problem in the theme "fishy"

To imitate the default behaviour of `fish(1)` `fishy.zsh-theme` includes a function to collapse the current working directory shown in the prompt from `/path/to/something` to `/p/t/something`. It calls `perl -pe` to do so. Unfortunately, as it is now the Perl one-liner breaks when the fed multibyte UTF-8 characters. This commit fixes that by forcing Perl to use UTF-8 I/O.

Before:

    dbohdan@bayes /m/D/D/�/�/testls

After:

    dbohdan@bayes /m/D/D/М/Р/test> ls

(Those are Cyrillic "М" and "P".)