Think about the overhead involved. Here is my brief analysis:
open(INFILE,"cat textfile |")
This opens a type of file handle commonly known as a pipe.
It spawns an additional process, complete with a duplicate set of environment variables and memory management requirements. The OS must now swap memory back and forth between Perl and cat.
Perl opens a file handle directly to the file. No other processes are started.
system("cat textfile | filter.pl")
Perl invokes the shell which invokes cat and another instance of Perl! Plus the shell still has to open a file handle for the output of cat / the input to filter.pl
Result: All three methods require a filehandle (aka a fileno, or a file descriptor) and two of the methods have the additional overhead of multiple processes. Use the second method and avoid all that.