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

<x><chapter><chapnum>1</chapnum></chapter><x><chapter><chapnum>1A</cha
+pnum>

<x><chapter><chapnum>1</chapnum></chapter><chapter><chapnum>1A</chapnu
+m>

<x><chapter>
<chapnum>1</chapnum>
...
<x><chapter>
<chapnum>1A</chapnum>
</chapter><x>

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

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