Your question here seems to imply an assumption. This might be: "Why is indexing beyond the end of the list legal for a slice when it's illegal for an array?

use strict;
use warnings;
use 5.014;  

my @arr = ();

if (exists $arr[0]) {
    say 'yes';
}
else {
    say 'no';
}

say $arr[0];

--output:--
no
Use of uninitialized value in say at 1.pl line 15.
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If the subscript indicates an element that would be beyond the end of the array, the corresponding value will be undef.

For comparison:

my @arr = (undef);

if (exists $arr[0]) {
    say 'yes';
}
else {
    say 'no';
}

say $arr[0];

--output:--
yes
Use of uninitialized value $arr[0] in say at 1.pl line 14.
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