I think this is referring to the fact that you can't use next to "just" jump out of a sub (or do or eval) - it needs to jump out of another block (which does not return a value) at a higher level. Compare this:

sub foo () { next }
foo
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with this:

sub foo () { next }
foo for 1
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The first dies with Can't "next" outside a loop block; the second runs successfully, but will warn when warnings are enabled.