I can't find anything in the documentation for IO::Socket or in the man pages

And yet IO::Socket::INET says:

               LocalAddr   Local host bind address      hostname[:port
+]
               LocalHost   Synonym for LocalAddr
               LocalPort   Local host bind port         <service>[(<no
+>)] | <no>
[download]

Perhaps you missed that. Seems pretty clear to me.

-- Randal L. Schwartz, Perl hacker
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