$ perl -Mre=debug -E " for( abcd ){  while(  m/./g){ say pos; } } "
Compiling REx "."
Final program:
   1: REG_ANY (2)
   2: END (0)
minlen 1
Matching REx "." against "abcd"
   0 <> <abcd>               |  1:REG_ANY(2)
   1 <a> <bcd>               |  2:END(0)
Match successful!
1
Matching REx "." against "bcd"
   1 <a> <bcd>               |  1:REG_ANY(2)
   2 <ab> <cd>               |  2:END(0)
Match successful!
2
Matching REx "." against "cd"
   2 <ab> <cd>               |  1:REG_ANY(2)
   3 <abc> <d>               |  2:END(0)
Match successful!
3
Matching REx "." against "d"
   3 <abc> <d>               |  1:REG_ANY(2)
   4 <abcd> <>               |  2:END(0)
Match successful!
4
Freeing REx: "."