It's quite possible that I'm being exceedingly dense, but how is this any different from just:
perl -E"say reverse<>" /etc/passwd

UPDATE- we only wanted it to work as described below-- with implicit reassignment of $_ to $a and also to $b.

It's not the same. BrowserUK's solution uses two regexes, each with a capture group. The capture groups implicitly assign to what would be the equivalents of "$a" and "$b". Since the point of this excercise was to save typing (potentially at the loss of clarity), BrowserUK didn't bother wasting precious keystrokes in explicitly assigning the result of the regex capture groups to an "$a" or "$b". Instead, he/she (I don't know BrowserUK personally) just let Perl handle those details. BrowserUK definitely meant what was posted ;~)