Variables are only available (by name) in the statement following the one in which they are declared. It allows the following to work.

my $var;
...
{
   my $var = $var;
   ...
}
[download]

So the key is to not refer to the new var by name or to put multiple statements in the if expression.

The only think that comes to mind is

if (
   sub {
      our @f; local *f = $_[0];
      (@f = some_code()) == 1 || (@f = some_other_code()) == 1
   }->(\my @f)
) {
   #...
}
[download]

Yuck.

This isn't a general purpose solution, but it works here:

if (my @f = do {
   my @f;
   ((@f = some_code()) == 1 || (@f = some_other_code()) == 1) ? @f : (
+)
}) {
   #...
}
[download]

Yuck.

Update: Added a missing pair of parens to the last snippet.