Assuming you want to replace password (in your example) with $usernewpwd, you need to make sure you preserve the "xdbns='jabber:iq:auth'>" and "<" parts, by including them in the replacement. So: 
s/xdbns='jabber:iq:auth'>.*?</xdbns='jabber:iq:auth'>$usernewpwd</g;

[download]
or 
s/(xdbns='jabber:iq:auth'>).*?(<)/$1$usernewpwd$2/g;

[download]
Note that I changed .* to .*?; this will cause it to prefer to stop matching at the first < instead of going on to a later < on the line.

Have you considered quoting issues? E.g. if a password actually contains a character like "<", what will whatever usually builds the file put there? And can you do it the same way to $usernewpwd, and adjust the regex to handle it? It might be better to look to one of the fine XML modules available on CPAN, than do it by hand.

Update: And as Boris says, you need to make sure to actually write out the changed lines to a new file.

In reply to Re: editing a file with regular expressions by ysth
in thread editing a file with regular expressions by gcazian

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.