Create JIRA sub-tasks automatically when issue created

You could so this easily using Jira Scripting Suite by using post function on issue creation transition on the project's workflow. Sample code:

from com.atlassian.jira.util import ImportUtils
from com.atlassian.jira import ManagerFactory
from com.atlassian.jira.issue import MutableIssue
from com.atlassian.jira import ComponentManager
from com.atlassian.jira.issue.link import DefaultIssueLinkManager
from org.ofbiz.core.entity import GenericValue;

# get issue objects
issueManager = ComponentManager.getInstance().getIssueManager()
issueFactory = ComponentManager.getInstance().getIssueFactory()
authenticationContext = ComponentManager.getInstance().getJiraAuthenticationContext()
subTaskManager = ComponentManager.getInstance().getSubTaskManager();
issueLinkManager = ComponentManager.getInstance().getIssueLinkManager()
customFieldManager = ComponentManager.getInstance().getCustomFieldManager()
userUtil = ComponentManager.getInstance().getUserUtil()

# define subtask
issueObject = issueFactory.getIssue()
issueObject.setProject(issue.getProject())
issueObject.setIssueTypeId("5") # normal subtask
issueObject.setParentId(issue.getId())

# set subtask attributes
issueObject.setFixVersions(issue.getFixVersions())
issueObject.setAffectedVersions(issue.getAffectedVersions())
issueObject.setPriority(issue.getPriority())
issueObject.setSummary("Auto created sub task- "+issue.getSummary())
issueObject.setAssignee(userUtil.getUserObject("joe"))

# Create subtask on JIRA 4.x
# subTask = issueManager.createIssue(authenticationContext.getUser(), issueObject)
# subTaskManager.createSubTaskIssueLink(issue.getGenericValue(), subTask, authenticationContext.getUser())

# Create subtask on JIRA 5 and higher
subTask = issueManager.createIssueObject(authenticationContext.getLoggedInUser(), issueObject)
subTaskManager.createSubTaskIssueLink(issue, subTask, authenticationContext.getLoggedInUser())

# Link parent issue to subtask   issueLinkManager.createIssueLink(issue.getId(),issueObject.getId(),10300,1,authenticationContext.getUser())

# Update search indexes
ImportUtils.setIndexIssues(True);
ComponentManager.getInstance().getIndexManager().reIndex(subTask)
ImportUtils.setIndexIssues(False)