sub func { return (0, 1, 2) }

if (1 and do { ($result) = func($param); $result }) {
   print "1\n";
} else {
   print "0\n";
}

if (1 and ($result) = func($param)) {
   print "1\n";
} else {
   print "0\n";
}
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outputs

0
1
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---

($result) = func($param)
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is a list assignment, and it happens to be in scalar context. List assignment in scalar context returns the number of elements returned by its RHS. (See Mini-Tutorial: Scalar vs List Assignment Operator.)

If func returns three values, the second snippet is equivalent to

if (... and do { ($result) = func($param); 3 })
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which is different than

if (... and do { ($result) = func($param); $result })
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As shown above, the difference will matter when func returns more than one value and the first is false.

---

You can avoid the do using a list slice.

if (... and $result = (func($param))[0])
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Scalar assignment returns its LHS.