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>