C:\>type test.pl

my ($switch, $proxy) = @ARGV;
$switch = ($switch eq '-p') ? 1 : 0;
$proxy = defined $proxy ? $proxy : 'undef';
print "Switch is '$switch', and proxy is '$proxy'\n";

C:\>perl test.pl

Switch is '0', and proxy is 'undef'

C:\>perl test.pl -p

Switch is '1', and proxy is 'undef'

C:\>perl test.pl -p http://proxy.foo.com/proxy.pac

Switch is '1', and proxy is 'http://proxy.foo.com/proxy.pac'

C:\>perl test.pl http://proxy.foo.com/proxy.pac

Switch is '0', and proxy is 'undef'

C:\>