The no warnings qw/syntax/; line seems unnecessary. Is there a reason for it?

For this program, that line doesn't serve any purpose. But then, neither do the two lines above - yet you aren't inquiring about them. The first six lines are generated whenever I type ^A-P (two chars), and I don't bother to remove the last line if it happens to occur in a program where it's unnecessary.

Trying to answer my own question I did re-implement your solution with simple tied scalars, and found no functional difference,

The fact that the entire word is replaced with identical characters didn't strike you as a functional difference? In this specific case, you could get away with using tied scalars if you also used /e, but if letters had to be replaced with a random letter enclosed in parenthesis, using a tied has would lead to:

    s/(\w)/($h{$1})/g;
[download]

while you would need something like:

    s/(\w)/"($s)"/eg;
[download]

which, while it has less characters, is less obvious, IMO.

Abigail