Hi guys,

I have solved the problem myself. If anyone needs help in future, my code is below:

public class LeadSourceFetcher{
//Function to carry out the operation when trigger fires
public static void LeadSourceFetch(List<Lead> leadsourcetrigger){
List<Lead_Source__c> temp = [Select id, where_hear_about, Domain__c,
Search_Cookie_source__c,Lead_Source__c from Lead_Source__c];
List<Lead_Source__c> newrecord = new List<Lead_Source__c>{};

//fetch the permission for users from user object
boolean ispermitted = [select Lead_Source_Trigger__c from User where id = :UserInfo.getUserId()].Lead_Source_Trigger__c;
//List<User> ispermitted = [Select id from User where Lead_Source_Trigger__c = true];
boolean flag = false;

//Trigger to on Lead object for given Criteria
for(Lead l:leadsourcetrigger){
//Iterate through the records for any match on required fields
for(Lead_Source__c ls:temp){
//if there is a match found with lead source object fire this condition
if((ls.where_hear_about_ == l.where_hear_about)&&
(ls.Domain__c == l.Domain__c)&&
(ls.Search_Cookie_source__c == l.Search_Cookie_source__c)){

//populate leadsource field of Lead object with leadsource field of Lead Source object
l.LeadSource = ls.Lead_Source__c;
l.lead_source_reference__c = ls.id;

//lupdate.add(l);
flag = true;
break;
}
}
//if there is no match found create a new record
if(!flag) {
//creating a new record on Lead Source and adding in object
newrecord.add(new Lead_Source__c(
where_hear_about = l.where_hear_about,
Domain__c = l.Domain__c,
Search_Cookie_source__c = l.Search_Cookie_source__c));
flag = true;
break;
}
}
//insert new record when there's no match
if(ispermitted){
insert newrecord;
}
//update lupdate;
system.debug('yes');
}