open (PIPE,"| read_three.pl") or die "Can't open pipe: $!";

to

open (PIPE,"| ./read_three.pl") or die "Can't open pipe: $!";

##</code><code>##

open (PIPE,"| perl read_three.pl") or die "Can't open pipe: $!";

##</code><code>##

C:\temp>perl write_ten.pl
Writing line 1
Read_three got: This is line number 1
Writing line 2
Read_three got: This is line number 2
Writing line 3
Read_three got: This is line number 3
Writing line 4
Writing line 5
Writing line 6
Writing line 7
Writing line 8
Writing line 9
Writing line 10
Wrote 3 lines of text