You can't do it easily with a single regex, but you can do it with a modest amount of code:

use strict;
use warnings;

while (<DATA>) {
    if (m|<list>| .. m|</list>|) {
        s/^(?!<list>)|(?<=<list>)/<item>/g;
        s-(?=</list>)|(?<!</list>)(?=\n)-</item>-g;
    }
    
    print;
}

__DATA__
The first quick brown fox jumps over the lazy dog.
<list>The quick brown fox jumps over the lazy dog.
The quick brown fox jumps over the lazy dog.
The quick brown fox jumps over the lazy dog.</list>
The last quick brown fox jumps over the lazy dog.
[download]

Prints:

The first quick brown fox jumps over the lazy dog.
<list><item>The quick brown fox jumps over the lazy dog.</item>
<item>The quick brown fox jumps over the lazy dog.</item>
<item>The quick brown fox jumps over the lazy dog.</item></list>
The last quick brown fox jumps over the lazy dog.
[download]