Nearly universally the bang for me, when the standard slash won't do. If that still doesn't help, it means the pattern is convoluted and calls for /x, which works best with paired delimiters (curlies for me). I tend to do something like

s{  foo  }{  bar  }x
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which nicely exposes the delimiters to skimming eyes. Add more whitespace if necessary. I'm not keen on splitting the pattern across lines, and prefer to avoid it if it isn't really necessary.

Makeshifts last the longest.