tlm,
Depends on your definition of evil and your perspective. Messing with the symbol table like this can certainly be useful. I think a more common version of call_me_only_once() for this would be:

sub foo {
    no warnings 'redefine';
    if ($cond1) {
        *foo = sub { ... };
    }
    elsif ($cond2) {
        *foo = sub { ... };
    }
    else {
        *foo = sub { ... };
    }
    goto &foo;
}
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The objective being to determine which version of foo() to use (perhaps XS, module, roll-your-own) only once so that subsequent calls are faster.