In Perl, what is omitted precisely when the variables are declared with omission?

what are declares with omission.pl

use strict;
use warnings;
use feature qw(say);

say "Firstly, what is returned with defined()?";

say "At line ", __LINE__ + 1, ", if it is defined, ",
    ( local $_ = defined 1 ) ? "true: '$_'" : "false '$_'";
say "At line ", __LINE__ + 1, ", if it is undef, ",
    ( local $_ = defined undef ) ? "true: '$_'" : "false '$_'";
say "At line ", __LINE__ + 1, ", but this false is 'defined false', so 'defined defined undef' is ",
    ( local $_ = defined $_ ) ? "true: '$_'" : "false '$_'";
say "At line ", __LINE__ + 1, ", What is 'defined false'?\nIt is the values that is NOT true but ",
    defined( local $_ = !1 ) ? "defined as: '$_'" : "undefined";
say '----';

say "Secondly, what is 'undefined'?";

say "At line ", __LINE__ + 1, ", of cource undef is ",
    defined( local $_ = undef ) ? "defined as: '$_'" : "undefined";
say "At line ", __LINE__ + 1, ", also () is ",    # comment for tidied code
    defined( local $_ = () ) ? "defined as: '$_'" : "undefined";
say "At line ", __LINE__ + 1, ", the nested () is also ",
    defined( local $_ = ( ( ( () ) ) ) ) ? "defined as: '$_'" : "undefined";
say "At line ", __LINE__ + 1, ", the list of empty list is also ",
    defined( local $_ = ( (), (), () ) ) ? "defined as: '$_'" : "undefined";
say "At line ", __LINE__ + 1, ", No matter how complicated, the empty lists is ",
    defined( local $_ = ( ( () ), ( (), () ), () ) ) ? "defined as: '$_'" : "undefined";
say '----';

say "Thirdly, how does scalar() work?";
my @defined = qw(foo bar);
my @omitted;
my @definedEmpty = ();

say "At line ", __LINE__ + 1, ", the undefined values with scalar() is ",
    defined( local $_ = scalar(undef) ) ? "defined as: '$_'" : "undefined";
say "At line ", __LINE__ + 1, ", even if that was a list of undef, scalar returns ",
    defined( local $_ = scalar( (undef) ) ) ? "defined as: '$_'" : "undefined";
say "At line ", __LINE__ + 1, ", it's also ",
    defined( local $_ = scalar( undef, undef, undef ) ) ? "defined as: '$_'" : "undefined";
say "At line ", __LINE__ + 1, ", No matter how complicated, the scalar() returns ",
    defined( local $_ = scalar( undef, ( (), undef ), () ) ) ? "defined as: '$_'" : "undefined";
say "At line ", __LINE__ + 2,
    ", because the list with scalar context returned the last value in itself ",
    defined( local $_ = scalar( 1, undef, ( (), undef ), () ) ) ? "defined as: '$_'" : "undefined";
say "At line ", __LINE__ + 2,
    ", if defined variables were set, it returns how many does it contain in the array.\nSo ",
    defined( local $_ = scalar(@defined) ) ? "defined as: '$_'" : "undefined";
say "At line ", __LINE__ + 1, ", if omitted, it returns ",
    defined( local $_ = scalar(@omitted) ) ? "defined as: '$_'" : "undefined";
say "At line ", __LINE__ + 1, ", it's also ",
    defined( local $_ = scalar(@definedEmpty) ) ? "defined as: '$_'" : "undefined";
say "At line ", __LINE__ + 1, ", HOWEVER, it returns ",
    defined( local $_ = scalar( () ) ) ? "defined as: '$_'" : "undefined";
say "Therefore, scalar() knows the value is defined or not";
say '----';

say "Before the conclusion, we have to confirm how do declares work.";

sub catch {
    my $line = ( caller() )[2];
    my @list = @_;
    do {
        local $_ = shift @list;
        say "At line $line, the variable is ", defined($_)
            ? "defined as: '$_'"
            : "undefined";
    } while (@list);
    return wantarray ? @_ : scalar @_;
}

say "Now we can catch the content of declares";

catch my $defined0 = 'defined';
catch my $defined1 = @defined;
catch my $empty    = ();
catch my $undef    = undef;
catch my $omitted;

catch my @array0 = @defined;
catch my @array1 = @omitted;
catch my @empty  = ();
catch my @undef  = undef;
catch my @omitted1;
say '----';

say "Finally, how does my() work?";
say "So, it returns ", ( local $_ = catch my $scalar0 = 'defined' ) ? "'$_'" : "undef";
say "So, it returns ", ( local $_ = catch my $scalar1 = @defined )  ? "'$_'" : "undef";
say "So, it returns ", ( local $_ = catch my $scalar2 = () )        ? "'$_'" : "undef";
say "So, it returns ", ( local $_ = catch my $scalar3 = undef )     ? "'$_'" : "undef";
say "So, it returns ", ( local $_ = catch my $scalar4 )             ? "'$_'" : "undef";
say '----';

say "So, it returns ", ( local $_ = catch my @array2 = @defined ) ? "'$_'" : "undef";
say "So, it returns ", ( local $_ = catch my @array3 = @omitted ) ? "'$_'" : "undef";
say "So, it returns ", ( local $_ = catch my @array4 = () )       ? "'$_'" : "undef";
say "But, it returns ",    # Because it's a defined array with one undef.
    ( local $_ = catch my @array6 = undef ) ? "'$_'" : "undef";
say "so, it returns ", ( local $_ = catch my @array5 ) ? "'$_'" : "undef";
say '----';

use Data::Dumper qw(Dumper);

say "The Dump of \$scalar2 is ", Dumper \$scalar2;
say "The Dump of \$scalar3 is ", Dumper \$scalar3;
say "The Dump of \$scalar4 is ", Dumper \$scalar4;
say "So, are they whole SAME?";
say '----';

use Devel::Peek;
say "I can say Yes! because with using Devel::Peek";
Dump $scalar2;
Dump $scalar3;
Dump $scalar4;

exit;