require function is used to load a seperate perl script or module in run time.If the file to be loaded is not in the directories listed in @INC array then full path of the perl file is to be passed in the require function .
ex . require "require.pl"; require "Mymodule.pm"; The location of perl file require.pl should be in directories listed in @INC.
otherwise give fullpath - require"/home/user/require.pl";
Once the perl file is loaded INC variable is updated with latest loaded perl file as key and its location as value .That's why before loading perl file require function checks if the file is already there in INC.If file is there then it'll not load the file. Source: CoolInterview.com