Perhaps you can't see it because you've got the statement wrong. It should be

($a,$b)=($b,$a+$b);
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It's a simple list assignment. $a gets the value of $b and $b gets $a+$b.

Incidently, it can be a bad idea to use $a and $b in Perl scripts as they are "special" variables because of their use in sorting.

Blessed Be
The Pixel