Yes with a properly named custom-function operating on an alias.

init $var1 => 'value1';  # pretty elegant, right?
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No, not at all. First, init() is a poor name which doesn't, by name, tell at all what it is doing. Second, the sub you wrote isn't at all ternary in any way, since it misses $_[2]. Third and last, function LIST doesn't tell at first glance that it is about conditional assignment to $_[0] from either $_[1] or $_[2] based on defined-ness of $_[0].