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sub get_code_ref_by_fqn {  # Fully qualified name
   my ($fqn) = @_;
...
   return undef if ref(\$glob_or_code) ne 'GLOB';
   return *$glob_or_code{CODE};
}

sub get_code_ref_by_fqn {  # Fully qualified name
   my ($fqn) = @_;
...
   return undef if !defined(&$fqn);
   return \&$fqn;
}

use 5.012;
use warnings;
...
say get_code_ref_by_fqn('Foo::Bar::x') // '[undef]';
say get_code_ref_by_fqn('Foo::Bar::y') // '[undef]';
say get_code_ref_by_fqn('Foo::Bar::z') // '[undef]';