in my $x = 10; the = is a scalar assignment which returns the assigned scalar

in my ($y) = 10; the = is a list assignment which returns the number of assigned elements¹

That's a crucial trick for iterators, where returned elements might be false (like 0 , '' or undef).

So

while ( ($x) = iterator() ) {
...
}
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will work as long as anything (i.e. no empty list) is returned, while

 
while ( $x = iterator() ) {
...
}
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can break if you don't use special magics like "0 but true".

UPDATE:

1) in scalar context!