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"gugus"                "gugus"
 ^^ here        and       ^^ here

"gugus"                "gugus"
 ^^ "gus"       and       ^^ "s"

"gugus"                "gugus"
 ^^ "gus" yes!            ^^ "s" no!
=> match fails,
   keep looking        => match succeeds

"gugis"
 ^^ here

"gugis"
 ^^ "gis"

"gugis"
 ^^ "gis" yes!
=> match fails

 v v v v v v  (everywhere)
" s t a r t "

 "start"
 | "tart"
...
 | | | | | ""
 v v v v v v
" s t a r t "

 "start"        yes => fails
 | "tart"       no  => succeeds
...
 | | | | | ""   no  => succeeds
 v v v v v v
" s t a r t "

$ perl -wMstrict -MData::Dump
while ( "start" =~ /(?!start)/pg ) {
...
("sta", "", "rt")
("star", "", "t")
("start", "", "")