By logical conclusion, this will mean that they are no longer valid in variable names

From the looks of the discussion right now this is not true. You will need to disambiguate that the _ is not part of a variable name with whitespace. So: my $baz = $foo_bar.$bar_foo becomes my $baz = $foo_bar _ $bar_foo. The whitespace on the left side of the _ is not optional.

update: s/around/on the left side of/