A module shipped with Perl v5.6.0 (*) that automatically does DeferredCompile
of any subs you place below the __DATA__ line in your code - see man SelfLoader
(*) I do not know about its availability in earlier releases of Perl
SelfLoader is definitely in 5.005. One can generally assume it's CORE, as earlier versions of perl have long since been put out to pasture everywhere they're found.