Okay, have you tried 'Super Search?'
I typed - IO::Socket - in the search box, and after some brief reading, found the following:

sockopt(OPT [, VAL])

Unified method to both set and get options in the SOL_SOCKET level. If
+ called with one argument then getsockopt is called, otherwise setsoc
+kopt is called.
[download]

a little follow up to perlman:PerlFunc , brought me here , which shook loose the following:

usage: setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL

And Finally, From a Super Search document found here I found out that more info might be available from the C man pages from which the Modules are based.
The options for setsockopt/getsockopt are in setsockopt(2).

C-.