for ($input eq "end")

Really? It runs the loop once, setting $_ to 1 or "" depending on $input, but you aren't using $_ in your loop. It probably doesn't do what you expect it to do.

As for the second choice - afterwards you read in the input, assign it to $input, then fall out of the loop.

I'd do something like:

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"}
}
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