This just sounds so close to what Module::Replace does that I'm wondering if we couldn't find a way to reuse that module for about the same thing. Perhaps a syntax like:

use Module::Replace;
Module::Replace::replace_with('IO::Socket::INET', new => sub {
  my $class = shift;
  $class = 'IO::Socket::INET6' if $class eq 'IO::Socket::INET';
  IO::Socket::INET::SUPER_new($class, @_);
});
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(Untested, partly 'cuz Module::Replace doesn't do this yet, but it's sooooo close.)