G'day Marshall,

I think the issue here is that the OP wrote "read a file", not "read from the keyboard". If the sentinel value is detected, then processing of the file should stop; however, I think it's reasonable to assume that if no sentinel value is detected, then processing of the file should stop at the end of the file.

$ alias perle
alias perle='perl -Mstrict -Mwarnings -Mautodie=:all -MCarp::Always -E
+'
[download]

Here's a file that simulates your keyboard input. I've added an extra line that is to be ignored:

$ cat > pm_11133571.dat
234
asdf
quit
ignore
[download]

That is processed (almost) identically to your keyboard input:

ken@titan ~/tmp
$ perle 'my $line; while ( (print "list of letters: "),$line=<>, $line
+ !~ /\s*quit|exit|q\s*$/i) { print "doing something with $line\n"}' p
+m_11133571.dat
list of letters: doing something with 234

list of letters: doing something with asdf

list of letters:
ken@titan ~/tmp
[download]

I said "almost" because, as you may have noticed, there's a dangling "list of letters: " prompt as the last line of output.

However, if there's no sentinel value, processing does not stop at the end of the file; instead, the code is stuck in a loop. Also, warnings are emitted. Both of those things are what ++ikegami predicted.

Consider this slightly different input file (without a sentinel):

$ cat > pm_11133571_2.dat
234
asdf
ignore
[download]

Here's what happens:

$ perle 'my $line; while ( (print "list of letters: "),$line=<>, $line
+ !~ /\s*quit|exit|q\s*$/i) { print "doing something with $line\n"}' p
+m_11133571_2.dat
list of letters: doing something with 234

list of letters: doing something with asdf

list of letters: doing something with ignore

Use of uninitialized value $line in pattern match (m//) at -e line 1, 
+<> line 3.
 at -e line 1, <ARGV> line 3.
Use of uninitialized value $line in concatenation (.) or string at -e 
+line 1, <> line 3.
 at -e line 1, <ARGV> line 3.
list of letters: doing something with
list of letters:
[download]

The screen output stopped there; I used Ctrl-C to get back to my shell prompt. Of course, I could've typed one of the sentinel values (quit|exit|q) but would you expect users to know what those values were; and even if they did, would you expect them to do this in the "production" environment you described.

Update: I started to compose my response before your post had any updates. After posting, I see you've added three updates. That's not a complaint about the updates; just advice on what I was responding to.