Strange. Taking your exact code from above and runnning here I get:

P:\test>type 381208-2.pl
#! perl -slw
#use strict;

use Benchmark qw[ cmpthese ];
use List::Util qw[ first ];

our @array = 1 .. 100_000;

cmpthese( -1, {
    func_10th => sub {
        first { length > 3 } @array;
    },
    iter_10th => sub {
        for ( @array ) {
            return $_ if length > 3;
        }
    },
});

cmpthese( -1, {
    func_last => sub {
        $_ = first { length > 5 } @array;
    },
    iter_last => sub {
        for ( @array ) {
            return $_ if length > 5;
        }
    },
});

P:\test>381208-2
            Rate func_10th iter_10th
func_10th 1196/s        --      -61%
iter_10th 3076/s      157%        --
            Rate func_last iter_last
func_last 22.6/s        --      -23%
iter_last 29.3/s       30%        --
[download]

Which indicates something about the difference in OS, but I'm not sure what.

I also wonder what context Benchmark calls the subs in? And is first optimised for the void context case?