Using the always-recommended use strict; use warnings; reveals one problem with your code:

Bareword "WNOHANG" not allowed while "strict subs" in use
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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:

#!/usr/bin/perl

# always uses these;
use strict;
use warnings;

# and detect errors 
use autodie;

pipe(READER, WRITE);
my $pid =fork();
if ($pid)
{
        print "In parent\n";
        close WRITE;
        my $val1 = 100;
        while(my $num = <READER>)
        {
                print "in while; $num\n";
        }
        close(READER);
        wait;
}
else
{
        print "In child\n";
        my $val2 = 150;
        print WRITE "$val2\n";
        close(WRITE);
        print "Child closed write\n";
}
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