[me@host]$ cat t2.pl
#!/usr/bin/perl -s
use strict;
use vars qw($a $b);

print "a: $a\n";
print "b: $b\n";
[me@host]$ perl t2.pl -a
a: 
b: 
[me@host]$ ./t2.pl -a
a: 1
b: 
[me@host]$