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