This has nothing to do with the prototype. The following produces the same warning.

for (qw(The quick brown fox)) {
   s/o/u/g;
}
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This doesn't produce any warning, but has side-effects. (I think. I can't remember the circumstances.)

for (1..5) {
   $_ = ...;
}
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You need to make a copy of the constants to modify them.

for (map "$_", qw(The quick brown fox)) {
   s/o/u/g;
}
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for (qw(The quick brown fox)) {
   my $s = $_;
   $s =~ s/o/u/g;
}
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for (qw(The quick brown fox)) {
   for (my $s = $_) {
      s/o/u/g;
   }
}
[download]

Update: Fixed a copy and paste error. The first two snippets were identical.