So I think it already counts past lexems and thus can determine the balance of "()", '[]' and "{}"

It can't currently.

If all the following conditions are true

All of the following satisfy your criteria, are valid and normal perl code, and would get a semicolon incorrectly inserted based on your criteria:

use softsemicolon;

$x = $a
   + $b;

$x = 1
    if $condition;

$x = 1 unless  $condition1
           && $condition2;
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The warning issued should be something like

I didn't ask what the text of the warning should be, I asked how the parser can determine when the warning should be issued.

the scheme proposed for recursive descent parsers

But perl uses an LR(1) parser, not a recursive descent parser.

Dave.