This is fairly easy to implement and wrap in a sub, even with the undefs. Once the sub is written you are back to having a one-liner. What would you like that this wouldn't do?

use strict;
use warnings;

sub compress {
  my $x;
  map {
    if (!defined($x)) {
      defined($_) ? ($x = $_) : ()
    } else {
      defined($_) && ($x eq $_) ? () : ($x = $_)
    }
  } @_;
}

my @aData=(qw(a a a a b c c a a d e e e e), undef, undef, qw(f g g));
my @aCompressed = compress @aData;
print "compressed: @aCompressed\n";

# outputs
a b c a d e undef f g

[download]

Or if you want to play golf (though others I'm sure can do better)

sub compress { my$x;map{defined($x)?(defined($_)&&($x eq$_)?():($x=$_)
+):defined($_)?($x=$_):()}@_;
}

[download]

Update: added golf

Update:: fixed mistake - undefs were strings.

In reply to Re: reduce like iterators by ELISHEVA
in thread reduce like iterators by LanX

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.