Ooops! The behavior appears to vary depending on how lines are executed, leading me to believe almut may be correct and that codeacrobat's observation may apply to 5.8.8 (and probably others). A caveat (or "confusion") follows. Compare using r instead of stepping:

>perl -d \pl_test\test{{.pl

Loading DB routines from perl5db.pl version 1.28
Editor support available.

Enter h or `h h' for help, or `perldoc perldebug' for more help.

main::(\pl_test\test{{.pl:5):
5:      my @i = (2..5);
  DB<1> {{ #
  DB<2> r
foo 3
foo 4
foo 5
foo 6
Debugged program terminated.  Use q to quit or R to restart,
  use o inhibit_exit to avoid stopping after program termination,
  h q, h R or h o to get additional info.
auto(-1)  DB<2> #

  DB<2>
...
[download]

BUT, stepping (entirely) through the script:

BUT:

>perl -d pl_test\test{{.pl

Loading DB routines from perl5db.pl version 1.28
Editor support available.

Enter h or `h h' for help, or `perldoc perldebug' for more help.

main::(pl_test\test{{.pl:5):
5:      my @i = (2..5);
  DB<1> {{ #
  DB<2> s
main::(pl_test\test{{.pl:7):
7:      for my $i (@i) {
auto(-1)  DB<2> #

  DB<2>
main::(pl_test\test{{.pl:8):
8:        $i ++;
auto(-1)  DB<2> #

  DB<2>
main::(pl_test\test{{.pl:9):
9:        print "foo";
auto(-1)  DB<2> #

  DB<2>
foomain::(pl_test\test{{.pl:10):
10:       print " $i\n";
auto(-1)  DB<2> #

  DB<2>
 3
main::(pl_test\test{{.pl:8):
8:        $i ++;
auto(-1)  DB<2> #

  DB<2>
main::(pl_test\test{{.pl:9):
9:        print "foo";
auto(-1)  DB<2> #

  DB<2>
foomain::(pl_test\test{{.pl:10):
10:       print " $i\n";
auto(-1)  DB<2> #

  DB<2>
 4
main::(pl_test\test{{.pl:8):
8:        $i ++;
auto(-1)  DB<2> #

  DB<2>
main::(pl_test\test{{.pl:9):
9:        print "foo";
auto(-1)  DB<2> #

  DB<2>
foomain::(pl_test\test{{.pl:10):
10:       print " $i\n";
auto(-1)  DB<2> #

  DB<2>
 5
main::(pl_test\test{{.pl:8):
8:        $i ++;
auto(-1)  DB<2> #

  DB<2>
main::(pl_test\test{{.pl:9):
9:        print "foo";
auto(-1)  DB<2> #

  DB<2>
foomain::(pl_test\test{{.pl:10):
10:       print " $i\n";
auto(-1)  DB<2> #

  DB<2>
 6
Debugged program terminated.  Use q to quit or R to restart,
  use o inhibit_exit to avoid stopping after program termination,
  h q, h R or h o to get additional info.
auto(-1)  DB<2> #

  DB<2>
Use `q' to quit or `R' to restart.  `h q' for details.
  DB<2> s
Use `q' to quit or `R' to restart.  `h q' for details.
  ...
[download]

WTF?