As far as I can tell, that wants to be an example child process to demonstrate what will happen when you close a filehandle associated with a long-running, sporadically outputting process (without STDOUT buffering). As if your child were this:

perl -le '$|=1; { print ++$i; sleep 1; redo; }'
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In the original post's terms this is similar to: open( my $fr_chld, "perl -le '...' |" )