$ cat sentinel_512.pl
#!perl

use 5.12.0;
use warnings;

my @in = qw{start middle end};
say map {
    state $i = 0;
    join($_, ($i++ == $#in ? q{<li class="last">} : q{<li>}), q{</li>});
} @in;

$ sentinel_512.pl
<li>start</li><li>middle</li><li class="last">end</li>