Ok, for command line args, by far the best way to do it (IMHO) is to use Getopt, but it will require you to change your syntax a little. Here is some example (sortof tested) code:

use Getopt::Std;

# set up informative strings
my $usage = "$0 version $VERSION by -= $AUTHOR =-
use -h for help";
my $help = "use $0 {-a | -o} <search1> <search2>
use $0 -a to "AND" your criteria
use $0 -o to "OR" your criteria
";

#when you call getopts, you pass a string and a hash ref.
# the string defines what switches you accept, and the hash
# gets filled in accordingly.  For instance, if someone put
# in a -h here, $options{h} would be set to 1 (true).
my %options;
getopts('huao', \%options);

# in this case, I'm assuming either and "and" or "or" is 
# required.
$options{u}=1 unless %options && ($options{a} || $options{o});

#it's more readable to break this into two tests 
#we need to make sure someone didn't put in -a and -o
$options{u}=1 if ($options{a} && $options{o});

# I haven't done it this way in a while, but IIRC
# anything not handled by getopts gets left in ARGV
# Therefore, we need to make sure that there are two
# options left (two criteria)
$options{u}=1 unless $#ARGV==2;

# now here we go.  for any of our test cases we would
# have set $options{u}, so if the command line was bad
# it will get caught and exited here.
if ($options{h} || $options{u}){
    print "$usage\n";
    print "$help\n" if ($options{h});
    exit 0;
}
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From here I hope it's pretty obvious. Either $options{o} or $options{a} is set, and the criteria just need to be assigned out of ARGV.

Like I said, the code is untested. It comes from my boilerplate new script template, but I've never tested this particular writeup.

Take some time to read the documentation for getopt::std and getopt::long, they are worth keeping in your perl arsenal