I assume your Find_First function modifies $repeat_flag according to some criteria. That's really the problem: it's unclear from this snippet that the subroutine call is controlling the loop. So here's one way to make it clearer.

OUTER:
while (1)
{
    while (($k, $v) = each %GRAMMAR)
    {
        last OUTER if Find_First($k, $v);
    }
}
[download]

I may have the "sense" of the test backwards; depending on what else you're doing, unless might make more sense. Also, OUTER is not really a descriptive label outside of this example. Pick something that makes the last read better.

In general, I've found that using a variable to control a loop is a habit from other programming languages. Given Perl's powerful loop constructs, if you find yourself using a loop control variable, you may want to see if there's a way to avoid it. That's not an absolute; sometimes it's clearer to just say "while ($keepgoing)" or something, but often a last or next to a well-chosen label can be better. There's an excellent example in the perlsyn document regarding nested loops just like this.

HTH