Why make things more complicated than they are? If the idea is to send commandline arguments to the server, then just send them (as you would in the parent) and don't read from STDIN — or maybe read from STDIN only if nothing is specified on the commandline, e.g. like this:

...
#the child process
else {
    unless (@ARGV) {
        # read a line from stdin, if no args given on command line
        my $line = <STDIN>;
        chomp $line;
        push @ARGV, split(' ', $line);
    }
    print $socket "$_\n" for @ARGV;
}
...
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