$ perl -wE'                     
open my $fh, "<", "foo" or die $!;
while (<$fh>) {
    print $_;
    sleep 1;
}
say "end";' &
[2] 15475
$ test1
test2
perl -pi -e 's/test/TEST/' foo
test3

$ test4
test5
test6
test7
test8
test9
test10
test11
end