It is named after the program lint which carries out a similar process for C programs. Before processing the check arguments, a standard list of checks is turned on.
Later options override earlier ones.
This option warns whenever an undefined subroutine is invoked. Any occurrence of any of these variables in your program can slow your whole program down. See the perlre manpage for details. Normally, Lint only checks the main code of the program together with all subs defined in package main. The -u option lets you include other package names whose subs are then checked by Lint.