if (defined $var1 || defined $var2)
if (defined $var1 && defined $var2)

##</code><code>##

if (defined ($var1 || $var2))
if (defined ($var1 && $var2))

##</code><code>##

./test.pl ""
Test 1: NOK
Test 2: OK
Test 3: OK
Test 4: NOK

##</code><code>##

#!/usr/bin/perl

use strict;
use warnings;

for my $i (1 .. 4) {
    my $result = (&successfulValidation ($i))? "OK": "NOK";
    print "Test ", $i, ": ", $result, "\n";
}

sub successfulValidation {
    my $test = shift;

    if ($test == 1) {
        return defined ($ARGV[0] || $ARGV[1]);

    } elsif ($test == 2) {
        return defined $ARGV[0] || defined $ARGV[1];

    } elsif ($test == 3) {
        return defined ($ARGV[0] && $ARGV[1]);

    } elsif ($test == 4) {
        return defined $ARGV[0] && defined $ARGV[1];
    }
}