Clever. It just needs one more goto to handle the empty case (no overrides) gracefully:

sub localize {
    my $real = pop;
    no strict 'refs';
    goto CALL unless @_;
    AGAIN: local *{shift@_} = sub { 'changed' };
    goto AGAIN if @_;
    CALL: $real->();
}
[download]

Even so, it's easier to understand than the nested-subroutines solution.

I wonder if anybody out there has got something even simpler.