$ cat sentinel_58.pl
#!perl

use strict;
use warnings;

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

$ sentinel_58.pl
<li>start</li><li>middle</li><li class="last">end</li>