Equivalent code, with comments 
use Data::Dump 'pp';

my @a = qw/2 1 3 4 5 4 3 3 4 5/;
my %seen;
my @result;

for ( @a ) {               #for each element in @a
    if ( not $seen{$_} ) { #if the element hasn't already been seen
        push @result, $_;  #add it to the result set
    }
    $seen{$_}++;           #note that the element's been seen
}

print join(",", @result), "\n";

pp \%seen;                 #{ 1 => 1, 2 => 1, 3 => 3, 4 => 3, 5 => 2 }

[download]

Each value in %seen is a count of how many times the key's been seen

Update: Removed a misleading comment & corrected a typo
Update^2: replaced say with print for perl<5.10 compatability (as suggested by ww)

Unless I state otherwise, all my code runs with strict and warnings

In reply to Re: Explain the code by FunkyMonk
in thread Explain the code by rose

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.