Filtering Your Own Output
Problem
You want to postprocess your program's output without writing a separate program to do so.

Solution
Use the forking form of open to attach a filter to yourself. For example, this will restrict your program to a hundred lines of output:

head(100);
while (<>) {
    print;
} 

sub head {
    my $lines = shift || 20;
    return if $pid = open(STDOUT, "|-");
    die "cannot fork: $!" unless defined $pid;
    while (<STDIN>) {
        print;
        last unless --$lines;
    } 
    exit;
}