I’d go with foreach my $var (1..5) every time, for the very same reason that “I don’t like to see the same global variable-name being used again and again.” Especially not, as in this case, one that you can’t see.
Okay, so it works okay now. Goody,goody, but then someone comes along and needs to make a truly-slight change ... which just happens to consist of some other control-structure that changes the meaning and value of $_. Now, they have to dance-around your program, making a bunch of source-code changes (entirely unrelated to what they wanted to do ...), just to do what you could well have done the first time. (And, to keep things realistic, let’s say they inadvertantly miss just one thing ...) Meh. That sort of thing is “just not worth the clever.”
Keep it simple ... keep it maintainable. I’m not laying-down any sort of commandment here, but it is trivially easy (as shown) to create a very nice, named, locally-scoped variable, associated with the loop and existing only within its scope, that contains the value of the loop index. That’s one of Perl’s nicest, most pragmatically useful features. (As is $_, yes, in small doses ...)
