Update: this bug has now been fixed for perl-5.10 (change #27547), and I think it is likely the fix will make it into one of the next maintenance releases (5.8.9 or 5.8.10) as well.

So you will be able to write:

  sub localise_and_call {
    my($fn, @args) = @_;
    my $stash = \%::;  # assuming main package
    local @$stash{@args} = (sub { "changed" }) x @args;
    $fn->(@args);
  }
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Hugo