telemachus ~ $ perl testre 
Perl 5.10.0 on linux:
$ARGV = {
          'WHEN' => undef
        };
$string = 'It is thyme';

telemachus ~ $ /usr/bin/perl testre 
Perl 5.10.0 on linux:
$ARGV = {
          'WHEN' => '12:34'
        };
$string = 'It is thyme';