There is no @main::a. You are benchmarking couple of no-ops.

use strict;
use warnings;
use Benchmark qw/ cmpthese timeit /;

our @b = my @a = map log, 1..1e6;
 
timeit( 1, 'print "\$#a = $#a\n";' );
timeit( 1, 'print "\$#b = $#b\n";' );

cmpthese( -2, {
    a => '[ map $_ * 4, @a ]',
    b => '[ map $_ * 4, @b ]',
});

cmpthese( -2, {
    1 => '[ map $_ * 4, @b ]',
    2 => '[ map $_ / (1/4), @b ]',
});

__END__

$#a = -1
$#b = 999999
        Rate          b          a
b     7.70/s         --      -100%
a 10680591/s 138780824%         --
    Rate    1    2
1 7.76/s   -- -28%
2 10.7/s  39%   --

[download]

In reply to Re^3: Should multiplication by integer be avoided in favour of division for performance reasons? (benchmark pitfalls) by vr
in thread Should multiplication by integer be avoided in favour of division for performance reasons? by vr

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.