Your DB counters are in the 30s (and skip backwards). My guess is it's something in the steps you are not showing us. Here's my SSCCE:

use strict;
use warnings;
use Test::More tests => 2;

my @matches;
$_ = '1223w3433.45+34';

@matches = (/\d\D*/g);
is $matches[0], 1, 'First time';
@matches = (/\d\D*/g);
is $matches[0], 1, 'Second time';
[download]

Which passes, as you would expect.

I'm trying to count digits in a scalar

Perhaps you could use tr///, see Quote Like Operators.

johngg@shiraz:~/perl/Monks$ perl -E '$_ = q{1223w3433.45+34}; say tr{0
+-9}{};'
12
[download]

I hope this is of interest.

... I'm trying to count digits in a scalar

That's what the OP says, but the  /\d\D*/g regex suggests misterperl is trying to count occurrences of some kind of digit-group pattern. If that's the case, I can, offhand, think of some  s/// approaches (in the vein of a  tr/// operation) that would do the trick as well as the  m// approach that misterperl seems to favor:

c:\@Work\Perl\monks>perl -wMstrict -le
"$_ = '+++1223w3433.45+34***';
 ;;
 my $ndg;
 ;;
 $ndg =()= /\d\D*/g;
 print qq{A: $ndg digit grps.;  m// change unpossible '$_'};
 ;;
 $ndg = do { (my $r = $_) =~ s/\d\D*//g };
 print qq{B: $ndg digit grps.;  s/// string unchanged '$_'};
 ;;
 $ndg = s/(\d\D*)/$1/g;
 print qq{C: $ndg digit grps.;  s/// string unchanged '$_'};
 ;;
 $ndg = s/\d\D*//g;
 print qq{D: $ndg digit grps.;  s/// string CHANGED '$_'};
"
A: 12 digit grps.;  m// change unpossible '+++1223w3433.45+34***'
B: 12 digit grps.;  s/// string unchanged '+++1223w3433.45+34***'
C: 12 digit grps.;  s/// string unchanged '+++1223w3433.45+34***'
D: 12 digit grps.;  s/// string CHANGED '+++'
[download]

My own preference would be for the  m// approach.

---

Give a man a fish:  <%-{-{-{-<

[~]$ perl -v

This is perl 5, version 16, subversion 3 (v5.16.3) built for x86_64-li
+nux
(with 1 registered patch, see perl -V for more detail)

Copyright 1987-2012, Larry Wall

Perl may be copied only under the terms of either the Artistic License
+ or the
GNU General Public License, which may be found in the Perl 5 source ki
+t.

Complete documentation for Perl, including FAQ lists, should be found 
+on
this system using "man perl" or "perldoc perl".  If you have access to
+ the
Internet, point your browser at http://www.perl.org/, the Perl Home Pa
+ge.

[~]$ perl -de1

Loading DB routines from perl5db.pl version 1.37
Editor support available.

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

main::(-e:1):   1
  DB<1> $_ = '1223w3433.45+34'

  DB<2> x /\d\D*/g
0  1
1  2
2  2
3  '3w'
4  3
5  4
6  3
7  3.
8  4
9  '5+'
10  3
11  4
  DB<3>
[download]

This is perl 5, version 16, subversion 3 (v5.16.3) built for x86_64-linux-thread-multi (with 33 registered patches, see perl -V for more detail)

Ok. Can you run the exact same commands I did above and see if it has the same or different output?

Further to hippo's post:

... something in the steps you are not showing us.

c:\@Work\Perl\monks>perl -wMstrict -le
"$_ = '1223w3433.45+34';
 ;;
 /\d\D*/g;
 printf qq{'$&' } while /\d\D*/g;
"
'2' '2' '3w' '3' '4' '3' '3.' '4' '5+' '3' '4'
[download]

Update: See what happens if the side-effect-producing, void context
    /\d\D*/g;
statement is changed to
    () = /\d\D*/g;
(i.e., if list context is imposed).

It's often the result of using the nonsensical if (/.../g)

$ perl -e'
   $_ = "abc";
   if (/\w/g) {
      CORE::say for /(\w)/g;
   }
'
b
c
[download]

I dont think I "contrived" anything. I did "R" and the first line was :

$_= '1223w3433.45+34';
[download]

then I did the "x". If the debugger works as you suggest, then it's not working right. It should interpret the expression correctly; if something isn't reset (which doesnt make sense ,since this happens on line 2) than it needs to get reset..

Larry?

But, you're correct it IS a debugger thing because the interpretation is correct outside dbg...

Please apply this single line and show us the result:

  DB<5> x $_='12a3b'; /\d\D*/g
0  1
1  '2a'
2  '3b'
  DB<6>
[download]

> But, you're correct it IS a debugger thing

Very unlikely.

And you've already been quick to blame Perl in the past.

Edit

We need a possibility to reproduce your problem before searching a hypothetical bug.

So please reproduce, and tell us the versions of perl and the debugger.

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}

$ perl -de0

Loading DB routines from perl5db.pl version 1.53
...
  DB<1> $_='1223w3433.45+34'

  DB<2> x /\d\D*/g
0  1
1  2
2  2
3  '3w'
4  3
5  4
6  3
7  3.
8  4
9  '5+'
10  3
11  4
  DB<3> p $]
5.028002
  DB<4>
[download]

I concur with the others that you most likely changed pos in a previous step.