First, I'm not the right person to be asking this question of. Whatever regex expertise I once had is well out of date. There is a whole bunch of stuff I've never done anyting with. However, this is my interpretation of the decidedly unclear documentation:

Three matches?

The (what appears to be called) zero-length switch assertion, appears to succeed, whenever the condition part succeeds; regardless of whether the yes pattern (or no pattern, if present) succeed.

So with 6 characters in your string, and a condition that restricts matching to every second character, the overall match succeeds 3 times, even if the yes pattern only matches at 2 of those positions. Hence your output.

Also, I notice the x modifier doesn't work with a conditional pattern:

It appears that you are breaking up an indivisible token with the whitespace you've used. This works:

C:\test>perl -M5.010 -w
my $str = 'bxAybz';

while ( $str =~ /
    (?(?{1})
      (b[xyz])
    )
/gx ) {
    say 'yes';
    say $1;
}
^Z
yes
bx
yes
bz
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