I am trying to understand split and its documentation. To do this I have implement split by using m{} and the variables @- and @+. By this I have found some problems

This script shows some of them:

use strict;
use warnings;
use 5.010;

use Data::Dump qw(dump dd ddx);
sub split_e { }

split_e( // );
# Warning: Use of uninitialized value $_ in pattern match (m//) at pm_
+1.pl ...

my @rv = split( //, '' );
# Warning: none

my $str = '1-10,20';
my $pat = '(-)|(,)';

@rv = split( $pat, $str );
warn dump @rv;    # (1, "-", undef, 10, undef , ",", 20)

@rv = $str =~ m{$pat}g;
warn dump @rv;    # ("-", undef, undef, ",")

while ( my $rv = $str =~ m{$pat}gc ) {
    for my $ix ( 0 .. 99 ) {
        if ( defined $-[$ix] ) {
            say sprintf '$ix= %d (%d,%d)<%s>', $ix, $-[$ix], $+[$ix],
              substr $str, $-[$ix], $+[$ix] - $-[$ix];
        }
    }
}

say "Strawberry Perl $^V";
say $^O;
__DATA__
output:
$ix= 0 (1,2)<->
$ix= 1 (1,2)<->
$ix= 0 (4,5)<,>
$ix= 2 (4,5)<,>
Strawberry Perl v5.30.1
MSWin32
[download]

Questions

Magic in split

This split_e( // ); gives the warning: Use of uninitialized value $_ in pattern match (m//) at pm_1.pl ... but this split( // ); does not.

Is there some magic in split indicate by the ‘/’s in “/PATTERN/ in the documentation?

Bug in the variables @- and @+

I find that the result from split, m{} and when using $-[$ix] $+[$ix] are inconsistent.

Split gives (1, "-", undef, 10, undef , ",", 20).

m{} gives ("-", undef, undef, ",").

Using $-[$ix] $+[$ix] does not include the undefs

Is this a bug in the last case?