If you need to modify already existing framework Perl module files or tt template files for customizations, you can do this in the $OTRS_HOME/Custom/ directory. Example: You need to modify Kernel/System/Service.pm to add an customization, copy Kernel/System/Service.pm to Custom/Kernel/System/Service.pm and modify it. This module will be used instead of the original Service.pm. This method also works for tt files. Just place them in Kernel/Output/HTML/$ThemeName, and they will be used if the theme is selected. If you use Apache + mod_perl or PerlEx, any Perl module files placed in this directory will only be loaded by your web server after you restarted it.