Strange behavior of "my" in "perl -d" and "perl -de"

johnnywang has asked for the wisdom of the Perl Monks concerning the following question:

While I was trying to learn something about perl internals and playing with Devel::Peek, I noticed the following behavior that I don't understand. The following simple script:

# devel.pl
use Devel::Peek;
my $a = undef;
Dump $a;
my $b = \$a; 
Dump $a; # note the reference count for $a
[download]

produces the correct reference count for $a when running "perl -d devel.pl", the output is as follows:

c:>perl -d devel.pl

Loading DB routines from perl5db.pl version 1.23
Editor support available.

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

main::(devel.pl:3):     my $a = undef;
  DB<1> n
main::(devel.pl:4):     Dump $a;
  DB<1> n
SV = NULL(0x0) at 0x1829ce0
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY)
main::(devel.pl:5):     my $b = \$a;
  DB<1> n
main::(devel.pl:6):     Dump $a;
  DB<1> n
SV = NULL(0x0) at 0x1829ce0
  REFCNT = 2             ###<----- correct count
  FLAGS = (PADBUSY,PADMY)
[download]

But if I do an interactive session using "perl -de 1", it produces the wrong count for $a:

c:>perl -de 1

Loading DB routines from perl5db.pl version 1.23
Editor support available.

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

main::(-e:1):   1
  DB<1> use Devel::Peek;

  DB<2> my $a = undef;

  DB<3> Dump $a;
SV = NULL(0x0) at 0x1aeccfc
  REFCNT = 1
  FLAGS = ()

  DB<4> my $b = \$a;

  DB<5> Dump $a;
SV = NULL(0x0) at 0x1aeccfc
  REFCNT = 1       ###<----- wrong reference count
  FLAGS = ()
[download]

However, if I do not use "my" in the above interactive session, all is well:

c:>perl -de 1

Loading DB routines from perl5db.pl version 1.23
Editor support available.

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

main::(-e:1):   1
  DB<1> use Devel::Peek;

  DB<2> $a = undef;

  DB<3> Dump $a;
SV = NULL(0x0) at 0x1aecc18
  REFCNT = 1
  FLAGS = ()

  DB<4> $b = \$a;

  DB<5> Dump $a;
SV = NULL(0x0) at 0x1aecc18
  REFCNT = 2        ###<--------correct here
  FLAGS = ()
[download]

What's going on here?

Comment on Strange behavior of "my" in "perl -d" and "perl -de" Select or Download Code

Replies are listed 'Best First'.
Re: Strange behavior of "my" in "perl -d" and "perl -de" by tlm (Prior) on Apr 15, 2005 at 22:37 UTC
The thing is that within "perl -d", each line *you type interactively* is effectively in it's own block. Try the following: `DB<1> my $x = 1 DB<2> p $x DB<3> $x = 1 DB<4> p $x 1` [download] Notice how the first initialization of `$x` did not persist, since that `$x` was a lexical, and hence confined to the scope of that line. The second initialization of `$x` did persist, however, because this `$x` is in fact `$main::x`: `DB<5> p $main::x 1` [download] Update: One other consequence of the fact that every line you type lives in its own block is that global variables that are implicitly localized to the enclosing scope (i.e. regexp-matching variables like $&, $', $`, $1, $2, $3...) will not survive from one line you type to the next. For example `DB<1> 'a' =~ /(.)/ DB<2> print $1 DB<3> 'a' =~ /(.)/ && print $1 a DB<4>` [download] Therefore, if you want to be able to use one of these variables over several lines, assign it to a package global: `DB<5> 'a' =~ /(.)/ && $g = $1 DB<6> print $g a DB<7>` [download] Update: Added some emphasis. the lowliest monk	[reply] [d/l] [select]
Re: Strange behavior of "my" in "perl -d" and "perl -de" by Fletch (Bishop) on Apr 15, 2005 at 23:40 UTC
Yup, and `perldoc perldebug` mentions this fact. `Note that the said "eval" is bound by an implicit scope. As a r +esult any newly introduced lexical variable or any modified capture b +uffer content is lost after the eval. The debugger is a nice environm +ent to learn Perl, but if you interactively experiment using material +which should be in the same scope, stuff it in one line.` [download]	[reply] [d/l] [select]
Re^2: Strange behavior of "my" in "perl -d" and "perl -de" by johnnywang (Priest) on Apr 16, 2005 at 06:02 UTC
I still don't understand, why then is the first experiment using "perl -d devel.pl" producing the correct reference count?	[reply]
Re^3: Strange behavior of "my" in "perl -d" and "perl -de" by Fletch (Bishop) on Apr 16, 2005 at 13:43 UTC
This warning applies only to code you type in interactively. Read all of the surrounding section in `perldoc perldebug` and it should make more sense.	[reply] [d/l]