$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 hErE
tOO. END
EOT
;;

$text =~ s[START(.+?)END]{ 
    'START' . join('', 
        map{ /^\\/ ? $_ : lc } split /(\s+)/, $1
    ) . 'END' 
}smge;;

print $text;;
I am IntErEsted in making changes to the following lines.
However,  the changes must be \CME made here.
There may be cases whErE
START changes need to be made here
too. END
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