u/muthm59

+= behaves differently after Data::Dump::pp. Is this a bug in Data::Dump or possibly in Perl core?

I encountered some unexpected behavior in a simple snippet where regex captures are assigned to variables, formatted via Data::Dump::pp, concatenated, and then incremented using +=.

use v5.10;
use Data::Dump qw( pp );

"9." =~ /^ (\d+) \. (\d*) $/x;
my $n = { i => $1, f => $2 };

say "pp output: ", pp $n;       # { f => "", i => 9 }

$n->{f} = "$n->{i}$n->{f}";     # "9"
$n->{f} += 2;                   # expected: 11

say "result: $n->{f} (expected: 11)";

Observed output:

pp output: { f => "", i => 9 }
result: 2 (expected: 11)

Instead of evaluating 9 + 2 = 11, the += 2 operation behaves as if the value of $n->{f} were numerically 0, resulting in 2.

I am not looking for a 'workaround' (I have already avoided it in the production code), but I would like to understand what causes this, and whether it warrants a bug report to Data::Dump or Perl core.

Observations

I tested several variations:

• using scalar variables instead of hash elements,
• using a direct assignment of constants instead of assigning the regex variables,
• removing the pp output.

The behavior changes as follows:

• With pp $n removed, the result is always correct.
• With scalar variables instead of hash elements, calling pp always causes the wrong value, regardless of whether the value originated from regex captures or constants.
• With hash elements, calling pp does not affect the result after constant assignment, but it does after assignment from regex captures.

My suspicion is that inspecting the variables inside Data::Dump::pp somehow alters their internal state, causing the numeric operation to behave differently.

Does this look like something that should be reported to Perl core, or to the Data::Dump maintainers?

Perl version breakdown

I ran a test script using several Perl versions that I have installed using perlbrew (all with Data::Dump version 1.25):

Perl v5.10.1 – v5.26.3: The bug does not occur.

Perl v5.28.3 – v5.44.0: The bug occurs.

Reproducing the bug

I have uploaded the full test matrix script here: failing_increment.pl, together with a Bash script test_with_available_perls that runs the test script for all Perls that are installed locally with perlbrew.

Thank you for your help!