Using the always-recommended use strict; use warnings; reveals one problem with your code:
Bareword "WNOHANG" not allowed while "strict subs" in use
Another is that only the parent process should wait for the child process; if doesn't make any sense to wait for anything in the child.
In addition, doing a loop with non-blocking waitpid is useless; a single call to a blocking waitpid makes more sense. Or if you have only one child process, wait is even simpler.
Finally, afer the fork there are two writers; one in the child, one in the parent process. You need to close the one in the writer too:
# always uses these;
# and detect errors
my $pid =fork();
print "In parent\n";
my $val1 = 100;
while(my $num = <READER>)
print "in while; $num\n";
print "In child\n";
my $val2 = 150;
print WRITE "$val2\n";
print "Child closed write\n";
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