An approach using nested substitutions is also possible, but there may be a bit too much double-negation going on here for comfort. 

use warnings;
use strict;

my $text = <<'EOT';
I am IntErEsted in making changes to the following lines.
However, START thE chAnges mUst bE \CME madE hErE. END
There may be cases whErE
START chAngEs  nEEd to be mAdE  \CME  hErE
tOO. END
EOT

$text =~
    s{ (?<= START) (.*?) (?= END) }
     { (my $replacement = $1) =~
         s{ (?<! \S) ([^\s\\] \S*) }
          { lc $1                  }xmsge;
       $replacement;
     }xmsge;

print qq{[[$text]] \n\n};

[download]
[[I am IntErEsted in making changes to the following lines.
However, START the changes must be \CME made here. END
There may be cases whErE
START changes  need to be made  \CME  here
too. END
]]

[download]

Good luck with the homework.

In reply to Re: A newbie's question in regex by AnomalousMonk
in thread A newbie's question in regex by eskay

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.