You can't use a scriptlet variable in Struts tags unless you put this variable to the value stack. But you'd better not use a scriptlet variable, but the variable value.
<%@ taglib prefix="s" uri="/struts-tags" %>
<%boolean bool_val=true;%>
real value : <%=bool_val%><br/>
expression evaluated value :
<s:set var="bool_val"><%=bool_val%></s:set>
<s:if test="#bool_val == 'true'">
TRUE
</s:if><s:else>
FALSE
</s:else>