I had to modify the code you recommended I run because it was giving me the following error:

Invalid version format (non-numeric data) at perlmonkII.pl line 7, nea
+r "package SpyHash "
syntax error at perlmonkII.pl line 9, near "package SpyHash {
[download]

Yepp, sorry, that's v5.14 syntax.

Then I ran this code on perl v.12.5 and I got

Assign to key CHLD at perlmonkII.pl line 34

So, it seems that Net::DNS does not modify $SIG{CHLD}. Or it does modify it so far behind the scenes that tie magic does not catch it.

Are you sure that your code does not mess with $SIG{CHLD}? Search all your sources for CHLD or $SIG.

Let's assume your code is clean. I added some calls to Data::Dumper to that code to verify the value of $SIG{CHLD}. Could you try that script? Comment out the tie %SIG,'SpyHash'; line and run again, so that we can see if tie has any effects.

#!/usr/bin/perl

use v5.12;
use strict;
use warnings;
use Data::Dumper;

package SpyHash;

use Tie::Hash; # for Tie::StdHash
use Carp qw( carp );

our @ISA=qw( Tie::StdHash );

sub STORE{
        my ($self,$key,$value)=@_;
        $key=~/^CH?LD$/ and carp "Assign to key $key";
        return $self->SUPER::STORE($key,$value);
    }

sub DELETE
    {
        my ($self,$key)=@_;
        $key=~/^CH?LD$/ and carp "Delete key $key";
        return $self->SUPER::DELETE($key);
    }

   1;


package main;

sub info
{
        my $where=shift;
        print $where,": ",Dumper($SIG{'CHLD'}),"\n";
}

info('start');
tie %SIG,'SpyHash';
info('after tie');
$SIG{'CHLD'}=sub { 'just a dummy' }; # this line should be reported
info('after set handler');
require Net::DNS; # Any module messing with $SIG{'CHLD'} should be rep
+orted from here
info('after require');
Net::DNS->import(); # ... or here
info('after import');
# For some extra paranoia, pretend to do some work with Net::DNS:
my $dns=Net::DNS::Resolver->new();
info('after creating instance');
my $reply=$dns->search('localhost');
info('after resolving localhost');
$reply=$dns->search('no.such.host.anywhere.invalid.');
info('after resolving junk');
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Alexander

Here's the output of your latest script:

start: $VAR1 = undef;

after tie: $VAR1 = undef;

Assign to key CHLD at perlmonkIII.pl line 42
after set handler: $VAR1 = sub { "DUMMY" };

after require: $VAR1 = sub { "DUMMY" };

after import: $VAR1 = sub { "DUMMY" };

after creating instance: $VAR1 = sub { "DUMMY" };

after resolving localhost: $VAR1 = sub { "DUMMY" };

after resolving junk: $VAR1 = sub { "DUMMY" };
[download]

The sources I posted in the first post were the entirety of the code that led to the oddity documented there. Could the tie itself be changing the behaviour seen there?

For example, if I comment out this single line from your latest script: tie %SIG,'SpyHash'; then my output becomes:

 
start: $VAR1 = undef;

after tie: $VAR1 = undef;

after set handler: $VAR1 = sub { "DUMMY" };

after require: $VAR1 = undef;

after import: $VAR1 = undef;

after creating instance: $VAR1 = undef;

after resolving localhost: $VAR1 = undef;

after resolving junk: $VAR1 = undef;
[download]