using the /c modifier (for not resetting pos if matching fails, see perlretut#Global matching) avoids an endless loop, but has strange effects, too:

This code

use warnings;
$\="\n";
$str = 'ABC';
print "-"x10;
print pos($str);
pos($str) = 1;
while ($str=~/.\G/gc) {
  print $&;
}
print pos($str);
 
#__DATA__
# pos($str) = pos($str);     
while ($str=~/.\G/gc) {
  print $&;
}
print "-"x10;
[download]

prints:

----------
Use of uninitialized value in print at /home/lanx/B/PL/PM/re_escG.pl l
+ine 5.
 
A
1
----------
[download]

but uncommenting line pos($str) = pos($str); prints

----------
Use of uninitialized value in print at /home/lanx/B/PL/PM/re_escG.pl l
+ine 5.
 
A
1
A
----------
[download]

weird!