Incrementing the count at the top of the loop is slightly cleaner:

my $size = 2;
my $i = 0;

while (@array) {
    ++$i;
    open my $out, '>', "$i.txt" or die $!;
    say {$out} $_ for splice @array, 0, $size;
}

say "$i files created.";
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The point here is not optimisation, but to ensure $i has the right value wherever it is used. For a short piece of code like this the difference is not much, but add a few conditionals inside the loop and suddenly the semantics for $i can become rather uncertain.