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

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

no strict 'refs';
my $before = exists $foo::{bar}; # false
my $bool = defined *{ 'foo::bar' }{CODE};
my $after = exists $foo::{bar}; # true

undef &{ 'foo::bar' };
no strict 'refs';
*{ 'foo::bar' } = sub {};

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

my $coderef = \&{ 'subroutine_name' }; 
#undef $$coderef; # typo ?
undef &{ $coderef };