Are the functions to be overridden all in the same package? If they are, you can do the localisation with a single hash slice:

  sub localise_and_call {
    my($fn, @args) = @_;
    my $stash = \%::;  # assuming main package
# I forgot the local here
    @$stash{@args} = (sub { "changed" }) x @args;
    $fn->(@args);
  }
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Update: oops, doesn't work when I add the "local" where it's supposed to go. I'm not sure why not though, maybe a bug.

Update: now reported as bug #38710.

Hugo