I would probably use a hash and the /e modifier: 
my %subst;
$subst{$_}++ for qw/foo bar baz/;
s/(\w+)/$subst{$_} ? subst($1) :  $1/ge;

[download]

Indeed you may also hide the hash lookup into a separate subst sub, if you have one: of course you're not required to, I wrote it like so just for illustrational purposes...

You may even put code refs into the hash to differentiate the behaviour for different (sets of) words. Briefly, the answer(TM) is 42^W/e!

In reply to Re: How to not match some words but match all the others by blazar
in thread How to not match some words but match all the others by set_uk

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.