Would I be correct in assuming that if(! open TST, ">>$var1"){ } would just be another way of doing unless(open TST, ">>$var1"){ }

Yes, you would.

If so, is this just another way of doing things or are there further benefits for using unless. I did once read that the code we use should be readable to humans and that the computers will still interpret our code. Is this a case of unless being easier to read than if not?

unless (foo) is just another way of writing if (not (foo)). It's up to you to decide what you find better readable.

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