#!/usr/local/bin/perl -w
    use strict ;
    use Switch ;
    use constant PI      => 3.141592653589793 ;
    use constant HALFPI  => ( PI / 2.0 ) ;   # half
    use constant TWOPI   => ( 2.0 * PI ) ;   # double

    print TWOPI."\n";
    print HALFPI."\n";

# directory is '/home'''''''''

    my $long = 4.567 ;  # radians
    my $convention = 'W' ;

    switch ( $convention ) {
      case ('E') {                       print "east longitude $long radians\n"; }
      case ('W') { $long = TWOPI-$long ; print "west longitude $long radians\n"; }
      else       { print "error: $convention not valid\n"; }
    };