new IO::Socket::INET->new(...)
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should simply be

IO::Socket::INET->new(...)
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If you have a buffering issue, just autoflush. For STDOUT, the following will do:

$|=1;
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Your real problem is that sysread(STDIN,$msg_out,100); blocks.

I started writing a solution. But honestly, it would be much simpler to avoid select() and use threads.