Because list assignments are parallel. A list of three items on the left expects three items on the right. Since there's only one, $var2 and $var3 remain undefined.

You can do what you want with:

my ($var1, $var2, $var3) = "Hello" x 3;
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Update: Oops - I read your question slightly wrong. To assign the same value three times, use

my ($var1, $var2, $var3) = map "Hello", 1..3;
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my ($var1, $var2, $var3) = ("Hello") x 3;
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