use Modern::Perl;

{local $/="\n\n";
    while (<DATA>) {
       s{<bindpass>welcome1</bindpass>}{<bindpass>welcome123</bindpass>} if m{^<binddn>cn=xyzzz</binddn>};
       print;
    }
}

__DATA__
<binddn>cn=abcde</binddn>
<bindpass>welcome1</bindpass>

<binddn>cn=abcde</binddn>
<bindpass>welcome1</bindpass>

<binddn>cn=xyzzz</binddn>
<bindpass>welcome1</bindpass>