Conceptually push and unshift are exactly symmetrical, along with pop and shift. Is there really a performance difference?

I tried it and wow: 

use strict;
use Benchmark;
use vars qw(%test);

%test = (
  push => sub {
    my @tmp;
    push @tmp, "bar" foreach 1..10000;
  },
  unshift => sub {
    my @tmp;
    unshift @tmp, "bar" foreach 1..10000;
  },
);

timethese (-5, \%test);

__END__
Benchmark: running push, unshift, each for at least 5 CPU seconds...
      push:  5 wallclock secs ( 5.46 usr +  0.05 sys =  5.51 CPU) @ 17
+.42/s (n=96)
   unshift:  8 wallclock secs ( 8.08 usr +  0.11 sys =  8.19 CPU) @  0
+.73/s (n=6)

[download]
Why is the one so much faster than the other?

In reply to RE (tilly) 3 (less?): A Not so Simple Append by tilly
in thread A Not so Simple Append by Limo

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.