Sometimes undef is initialized and sometimes not when hash values are fed to grep

[leszekdubiel]

Hello Perl Monks!

In perl5 there is a strange behaviour that hash values are sometimes initialized to undef and sometimes not. See this example:

 
#!/usr/bin/perl -CSDA

use utf8;
use Modern::Perl;
no warnings qw{uninitialized numeric};
use Data::Dumper;

my %h = (a => 'alfa', b => 'beta'); 

print "\n\nfirst try:\n";
print "\t>>$h{a}<<\n";
print "\t>>$h{XXX}<<\n";
print "\t>>$h{b}<<\n";
print "after first:\n", Dumper(\%h); 

print "\n\nsecond try:\n";
print map { "\t>>$_<<\n" } $h{a}, "mytext" . $h{YYY} . "after", $h{b};
print "after second:\n", Dumper(\%h); 


print "\n\nthird try:\n";
print map { "\t>>$_<<\n" } $h{a}, $h{ZZZ}, $h{b}; 
print "after third:\n", Dumper(\%h); 



=====================

OUTPUT: 



first try:
    >>alfa<<
    >><<
    >>beta<<
after first:
$VAR1 = {
          'a' => 'alfa',
          'b' => 'beta'
        };


second try:
    >>alfa<<
    >>mytextafter<<
    >>beta<<
after second:
$VAR1 = {
          'a' => 'alfa',
          'b' => 'beta'
        };


third try:
    >>alfa<<
    >><<
    >>beta<<
after third:
$VAR1 = {
          'b' => 'beta',
          'ZZZ' => undef,             <<<<<<------------ ?????? 
          'a' => 'alfa'
        };

Hash values are only used to form strings. Why sometimes they got initialized to "undef" and sometimes not? Does Raku (Perl6) has similiar behaviour?

This is what I asked in autovivification context: https://www.perlmonks.org/?node_id=11110437.