it means:

use strict;
use warnings;

my @array = (0, 1, 2);

untouched ($array[3]);
print exists $array[3] ? "Element exists" : "No element";
touch ($array[3]);
print "\n", exists $array[3] ? "Element exists" : "No element";

sub untouched {
}

sub touch {
    $_[0] = 1;
}
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Prints:

No element
Element exists
[download]

because the element at index 3 didn't exist until it was referenced in touch.

Update to answer actual question