I did it myself before peeking at the above answers, and came up with something that avoids grep and seems pretty concise to me, here it is: 
use strict;
use warnings;

my @big = qw(a b c d e f g h i);
my @small = qw(f c d g);
my %small; $small{$_}=1 for @small;

my @keys_wanted = @small;
foreach ( @big ) {
    push @keys_wanted, $_ unless $small{$_};
}

print "@keys_wanted";

[download]
This doesn't remove dupes though (see ikegami's comments above), and I'm not sure if it can be simply modified to do so.

In reply to Re: Keeping order, no duplicate and the complete keyset by tphyahoo
in thread Keeping order, no duplicate and the complete keyset by Ace128

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.