changing 1; to $_=1; in a module is not going to do anything significant,
unless $_ is not scoped to the module and something else using
the module expects $_ to be undefined initially. Your other example
is not a useless use of constant in void context
I'm not sure it's even in void context, but I'll take your word for that.
It's not useless because it's part of a larger, more meaningful statement,
if the remainder of the line consisted of only numeric operations, numbers,
(variables) and semi-colon, that would be useless use... and safe to muck with.
perl -pew "s/\b;([mnst])/'$1/g"