foreach loops sets $_ to refer to the current value in the list which is being iterated. In this case, the list consists solely of $b.

Your expression is equivalent, but it's not an acceptable answer since the question was about removing the need for using $b twice.

In reply to Re^3: Shortcut operator for $a->{'b'}=$b if $b; by ikegami
in thread Shortcut operator for $a->{'b'}=$b if $b; by rokadave

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