An important exception to the preceding rules is the empty string. You can't use ", as an empty string doesn't interpolate anything. It doesn't contain a literal quote or brace either, so the previous rules call for it to be written like so:
$error_msg = '';
But that's not a good choice. In many display fonts, it's far too easy to mistake '' (single-quote, single-quote) for " (a lone double-quote), which means that you need to apply the second rule for non-interpolated strings, and write each empty string like so, preferably with a comment highlighting it:
$error_msg = q{};
# Empty string
Also see the "Constants" guideline later in this chapter.