You clearly see that it does match, so why do you keep saying it doesn't match in response to my explanation?

1st Pass: It thinks it hasn't matched yet because pos == 0, so it matches, setting pos = 0.
2nd Pass: It thinks it hasn't matched yet because pos == 0, so it matches, setting pos = 0.
3rd Pass: It thinks it hasn't matched yet because pos == 0, so it matches, setting pos = 0.
...

...no repeated matches like in list context.

The while isn't there for decoration. It's the one doing the repeating.

you still haven't looked at the code I posted. With pos==0 it doesn't match, but there are two different kind of undefined pos (one after the variable is initialized, and one after the first match) which is evidently a bug!

  DB<168> $str="abcd"
 
  DB<169> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<170> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
0::
  DB<171> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<172> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<173> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<174> pos($str)=0
 
  DB<175> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
0::
  DB<176> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<177> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<178> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<179> pos($str)=undef
 
  DB<180> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<181> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<182> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
[download]

Cheers Rolf

you still haven't looked at the code I posted. With pos==0 it doesn't match

I chose to explain why it matches endlessly. Will you stop telling me it doesn't match. I'm not interested in that case.

which is evidently a bug!

The bug is in your code. So what if Perl behaves oddly when provided with irrational inputs. Fix the bug in your code and it's a non-issue.