use strict;
use warnings;

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

__END__
Example run:
C:\Users\xxx\Documents\PerlProjects\Monks>perl commandloopX.pl
list of letters: 234
doing something with 234

list of letters: asdf
doing something with asdf

list of letters: quit

C:\Users\xxx\Documents\PerlProjects\Monks>

##</code><code>##

while ( (print "list of letters: ") and defined ($line=<>) and $line !~ /\s*quit|exit|q
+\s*$/i)

##</code><code>##

use strict;
use warnings;

my $line;
while (1) {
   print "list of letters: ";
   defined( $line = <> )
      or last;

   $line !~ /\s*quit|exit|q\s*$/i
      or last;

   # Do something with $line.
}
__END__
C:\Users\xxxx\Documents\PerlProjects\Monks>perl commandloopX3.pl
list of letters: adf
list of letters: 123
list of letters: q