-s   enables rudimentary switch parsing for switches on
            the command line after the program name but before
            any filename arguments (or before a --).  Any switch
            found there is removed from @ARGV and sets the
            corresponding variable in the Perl program.  The
            following program prints "1" if the program is
            invoked with a -xyz switch, and "abc" if it is
            invoked with -xyz=abc.

                #!/usr/bin/perl -s
                if ($xyz) { print "$xyz\n" }

##</code><code>##

#!/usr/bin/perl -ws
# minus-ess.pl

use strict;

# if we run it ./minus-ess.pl -switch "foo" it should be defined. 
my $switch = "hooray" unless (defined $switch);

print $switch;

##</code><code>##

Global symbol "$switch" requires explicit package name at minus-ess.pl line 7.