Thanks for your answer. Here's a more concrete example to show the problem.

$ cat -n tt
     1  #!/usr/bin/perl
     2
     3  my $perl = <<'EOF';
     4  sub {
     5  my $x = 123;
     6  $x += shift;
     7  print "x is $x\n"
     8  }
     9  EOF
    10
    11  my $cf = eval $perl;
    12
    13  $cf->(4);

$ perl -d tt

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

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

main::(tt:3):   my $perl = <<'EOF';
main::(tt:4):   sub {
main::(tt:5):   my $x = 123;
main::(tt:6):   $x += shift;
main::(tt:7):   print "x is $x\n"

DB<8> s
main::(tt:11):  my $cf = eval $perl;

DB<8> s
main::((eval 20)[tt:11]:7):     ;
[download]

Now we are inside the eval, and we can see the code...

DB<8> p $#{"_<(eval 20)[tt:11]"}
7

DB<9> l (eval 20)
1       sub {
2:      my $x = 123;
3:      $x += shift;
4:      print "x is $x\n"
5       }
6 
7==>    ;
[download]

But as soon as we step out of the eval, the code is gone...

DB<10> s
main::(tt:13):  $cf->(4);

DB<10> p $#{"_<(eval 20)[tt:11]"}
-1
[download]

And yet, there are other evals out there, for the code stays around...

DB<11> p join "\n", map { "$_   $#{$_}" } grep /_<\(/, keys %::
_<(eval 20)[tt:11]   -1
_<(eval 22)[/usr/lib/perl5/5.8.8/SelfLoader.pm:38]   6
_<(eval 11)[/usr/lib/perl5/5.8.8/SelfLoader.pm:111]   1
_<(eval 12)[/usr/lib/perl5/5.8.8/SelfLoader.pm:38]   40
_<(eval 5)[/usr/lib/perl5/5.8.8/Term/ReadLine.pm:306]   -1
_<(eval 21)[/usr/lib/perl5/5.8.8/SelfLoader.pm:38]   10
_<(eval 23)[/usr/lib/perl5/5.8.8/SelfLoader.pm:38]   20
_<(eval 26)[/usr/lib/perl5/5.8.8/perl5db.pl:628]   4

DB<13> f eval 22
Choosing (eval 22)[/usr/lib/perl5/5.8.8/SelfLoader.pm:38] matching `ev
+al 22':
1       package readline; sub F_PreviousHistory {
2:          &get_line_from_history($rl_HistoryIndex - shift);
3       }
4 
5 
6       ;
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I no longer can get to my eval'd code...

DB<14> f eval 20
Choosing (eval 20)[tt:11] matching `eval 20':


DB<15> l
[download]

And neither can the debugger...

DB<15> s
main::CODE(0x97b7d28)((eval 20)[tt:11]:2):
2:

DB<15> 
main::CODE(0x97b7d28)((eval 20)[tt:11]:3):
3:

DB<15> 
main::CODE(0x97b7d28)((eval 20)[tt:11]:4):
4:

DB<15> 
x is 127

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.
[download]

So the question remains, why does my eval'd source code get discarded, while other eval'd source code stays in memory? And what can I do to convince perl to keep my eval'd source code?