Dear fellow Monks,

I have an interesting example code showing that autovivification could happen accidentally, even hidden for the user, when hash slices are passed to a user defined subroutine, but not when passed to a built-in function. How (and why) does it happen? 

sub mydump($) {
    print Data::Dumper->Dump( [ $_[0] ], ['*hash'] );
}

sub myjoin($@) {
    return join @_;
}

my %hash;
my $s1 = join( ':', @hash{qw(key)} );   # line no. 12
mydump( \%hash );
my $s2 = myjoin( ':', @hash{qw(key)} );
mydump( \%hash );

[download]
Output is: 
$ perl -w ./hashslices.pl
Use of uninitialized value within %hash in join or string at ./hashsli
+ces.pl line 12.
%hash = ();
%hash = (
          'key' => undef
        );

[download]

In reply to Autovivification and hash slices by norbert.csongradi

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