I'll start with...

$all = ~s/[.|]+/|/g;
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Have a look at perlop for details, but ~ is bitwise negation (so ~1 is 4_294_967_294). The s/// operator returns the number of replacements done, so that's what the ~ would be operating on. This is what happens:

1. The s/// replacement is performed (on the contents of $_, see perlvar).
2. The number of replacements that were done is bitwise negated.
3. That's assigned to $all.

The $all =~ s/[.|]+/|/g; syntax is one I'm guessing you're familiar with. In that case, the substitution is done on the variable $all.

Update: Oops, I answered the wrong question.