Roy Johnson and hardburn are both wrong. Both || and or impose scalar context on their left operand. The difference is that or has a lower precedence than = so between the two forms, only the one using or was written correctly.

@foobar = foo() || warn;

@foobar
  = ( scalar( foo() )
      || warn );
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@foobar = foo() or warn;

scalar( @foobar = foo() )
  || warn;
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