Your suggestion is insightful. We need to manipulate symbol table carefully. Though I was under the impression that the following causes autovivification, it doesn't: 

my $bool = defined &{ 'foo::bar' };

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c.f. 
my $namespace = do { no strict 'refs'; \%{"foo::"} };
my $bool = exists $namespace->{bar} && defined *{ $namespace->{bar} }{
+CODE};

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In fact, the following causes autovivification: 
no strict 'refs';
my $before = exists $foo::{bar}; # false
my $bool = defined *{ 'foo::bar' }{CODE};
my $after = exists $foo::{bar}; # true

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I misunderstood defined &{ 'foo::bar' } cause autovivification. That's why you think defined &{ 'foo::bar' } is safe, right?

I agree the name of the subroutine has to be undefined. We are allowed to avoid "no warnings redefine" when we use undef &{ 'foo::bar' }. 
undef &{ 'foo::bar' };
no strict 'refs';
*{ 'foo::bar' } = sub {};

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c.f. 
no strict 'refs';
no warnings 'redefine';
*{ 'foo::bar' } = sub {};

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By the way, the following should work: 
my $coderef = \&{ 'subroutine_name' }; 
#undef $$coderef; # typo ?
undef &{ $coderef };

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Thanks for your comment :)

In reply to Re^2: undef/defined &{$name} while "strict refs" by anazawa
in thread undef/defined &{$name} while "strict refs" by anazawa

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