If my RE is too long for a single line but rather simple, I just use /x without a comment within the RE (maybe there's one before, e.g

# replace all <a href="link"...>linktext</a>
# with [url="link"]linktext[/url]
$var =~
  s~<a \s+ href = "([^"]+)" [^>]* >
      (.+?)
      </a>
  ~\[url="$1"\]$2\[/url\]~gsx;
[download]

(Well, I know there are better ways to parse html than with REs, but it was the first example that came into my mind)

I only use g, s, m when really needed and not as default options.