$ perl -Mre=debug -le "$f=123; $f=~/^.*/g; warn pos $f; $f=~ /\d/g; warn pos $f; "
Compiling REx "^.*"
Final program:
   1: BOL (2)
   2: STAR (4)
   3:   REG_ANY (0)
   4: END (0)
anchored(BOL) minlen 0
Compiling REx "\d"
Final program:
   1: DIGIT (2)
   2: END (0)
stclass DIGIT minlen 1
Matching REx "^.*" against "123"
   0 <> <123>                |  1:BOL(2)
   0 <> <123>                |  2:STAR(4)
                                  REG_ANY can match 3 times out of 2147483647...
   3 <123> <>                |  4:  END(0)
Match successful!
3 at -e line 1.
Warning: something's wrong at -e line 1.
Freeing REx: "^.*"
Freeing REx: "\d"

$ perl -Mre=debug -le "$f=123; $f=~/^.*/; warn pos $f; $f=~ /\d/; warn pos $f; "
Compiling REx "^.*"
Final program:
   1: BOL (2)
   2: STAR (4)
   3:   REG_ANY (0)
   4: END (0)
anchored(BOL) minlen 0
Compiling REx "\d"
Final program:
   1: DIGIT (2)
   2: END (0)
stclass DIGIT minlen 1
Matching REx "^.*" against "123"
   0 <> <123>                |  1:BOL(2)
   0 <> <123>                |  2:STAR(4)
                                  REG_ANY can match 3 times out of 2147483647...
   3 <123> <>                |  4:  END(0)
Match successful!
Warning: something's wrong at -e line 1.
Matching REx "\d" against "123"
Matching stclass DIGIT against "123" (3 bytes)
   0 <> <123>                |  1:DIGIT(2)
   1 <1> <23>                |  2:END(0)
Match successful!
Warning: something's wrong at -e line 1.
Freeing REx: "^.*"
Freeing REx: "\d"

$