XSLT variable eval

Hi, All

I have some XML, one fragment for sample:
...
    <btr:root xmlns:btr="http://xxx.xxx.xxx" xmlns="http://www.w3.org/1999/xhtml">
        <input type="text" name="username" readonly="yes">
            <btr:attribute name="username" as="value"/>
        </input>
    </root>
...

As the result i want to obtain:
<input type="text" name="username" readonly="yes" value="some_value"/>

"some_value" - parameter for the XSL-transform.
------
<xsl:stylesheet version="1.0" encoding="koi8-r"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:btr="http://xxx.xxx.xxx"
  xmlns:xhtml="http://www.w3.org/1999/xhtml">
    
    <xsl:param name="username"/>
    <!-- .... -->
    
    <xsl:template match="btr:attribute">
      <xsl:attribute name="{@as}"><xsl:value-of select="${@name}"/></xsl:attribute>
                                                       ^^^^^^ - how eval this variable?
    </xsl:template>

    <!-- .... -->
</xsl:stylesheet>
--------
I use LibXSLT in Perl.
sorry, my english is bad :(
--
Best regards,
Andrey V. Elsukov