Something like this:
<xsl:choose>
<xsl:when test="$var = 'subdomain.mydomain.com'">
<xsl:call-template name="one.action"/>
</xsl:when>
<xsl:otherwise>
<xsl:call-template name="another.action"/>
</xsl:otherwise>
</xsl:choose>
Unless by "contains" you mean "contains as a substring", in which case change the test to contains($var, 'subdomain.mydomain.com').
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference