while (1) {
    local $| = 1;
    print "add, del, prn, end? ";
    my $answer = <>;
    chomp $answer;
    if ($answer eq "add") { ... }
    elsif ($answer eq "del") { ... }
    elsif ($answer eq "prn") { ... }
    elsif ($answer eq "end") {last;}
    else {print "Unknown command '$answer'\n"}
}