Re: while(<>) { ... } considered harmful

[dws]

Since   map { substr($_, 1, 1) = "foo" } qw(bar baz); and   grep { substr($_, 1, 1) = "foo" } qw(bar baz); both exhibit the problem you've observed, perhaps the advice might better be phrased

Don't write to $_ from within map { } or grep { }

local $_; is one way to avoid writing to it, since the 'it' is now different.

Update: Hm.. It's a bit worse than that.   foreach ( qw(foo) ) { $_ = 1 } also exhibits the problem.