It's meaningless in Perl, except as an anecdote, because the virtual machine has so many branches at the machine code level no matter what code you write, and no additional expensive penalty for jumping around in Perl code.
It's not just the jumping you save on, it's also the reduced testing. Duff's device does two things: it does loop unrolling, and it uses a neat/tricky way of dealing with the border case of a loop unrolling technique. Loop unrolling pays because of not having to evaluate a condition. Here's a benchmark showing duff's device to be significant faster than a plain loop (yes, the work done is contrived, but that's not the point): 
#!/usr/bin/perl

use strict;
use warnings;

use Benchmark qw /timethese cmpthese/;

our ($x, $y);
our  $N = shift || 100;

cmpthese -1  => {
    noduff   => '$x = 0;
                 my $n = $N;
                 while ($n --) {$x ++}',
    duff     => '$y = 0;
                 my $n = int (($N + 7) / 8);
                 goto "LABEL" . ($N % 8);
                 {   LABEL0: $y ++;
                     LABEL7: $y ++;
                     LABEL6: $y ++;
                     LABEL5: $y ++;
                     LABEL4: $y ++;
                     LABEL3: $y ++;
                     LABEL2: $y ++;
                     LABEL1: $y ++;
                     redo if -- $n > 0
                  }',
};

die "Unequal \$N == $N; \$x == $x; \$y == $y.\n" unless $N == $x && $N
+ == $y;

__END__
          Rate noduff   duff
noduff 39384/s     --   -42%
duff   68267/s    73%     --

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Abigail

In reply to Re: Duff's Device by Abigail-II
in thread Duff's Device by davido

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