Hi Anonymous Monk, Here is one way to do it. You have done couple of mistakes, you have not used the paranthesis properly for ?=$s2 and for your requirement, 'g' option modifier is not necessary. You have to take a look at perlre.

$str1='
<x><chapter>
<chapnum>1</chapnum>
</chapter>
<x><chapter>
<chapnum>1A</chapnum>
</chapter>
<chapter>
<chapnum>1B</chapnum>
</chapter>
<x><chapter>
<chapnum>1</chapnum>
</chapter>
<chapter>
<chapnum>3</chapnum>
</chapter>
<x><chapter>
<chapnum>1A</chapnum>
</chapter><x>';

$s1='<x><chapter>\s*<chapnum>1</chapnum>';
$s2='<chapter>\s*<chapnum>1A</chapnum>';

$str1 =~s#($s1)((?:(?!<x>).)*)<x>(?=$s2)#$1$2$3#si;

print $str1;

output:
-------
<x><chapter>
<chapnum>1</chapnum>
</chapter>
<chapter>
<chapnum>1A</chapnum>
</chapter>
<chapter>
<chapnum>1B</chapnum>
</chapter>
<x><chapter>
<chapnum>1</chapnum>
</chapter>
<chapter>
<chapnum>3</chapnum>
</chapter>
<x><chapter>
<chapnum>1A</chapnum>
</chapter><x>
[download]

update: Removed first ?:, now exactly matches your output.