use v5.12; use warnings; use Data::Dump 'dd'; $_ = 'aa'; s/(?=(a))(?{say "\$1='$1' \@-=(@-) pos=",pos})/b/g; dd [$1]; dd \@-; dd $_; __END__ $1='a' @-=(0 0) pos=0 $1='a' @-=(0 0) pos=0 $1='a' @-=(1 1) pos=1 $1='a' @-=(1 1) pos=1 ["a"] [1] "baba"