chomp(my $in1 = $ARGV[0] || '');
if (defined $in1) {
    #Validate;
}

##</code><code>##

my $in1 = $ARGV[0] || 'user-did-not-pass-an-argument');
print $in1;

##</code><code>##

perl -w myscript.pl # no argument
perl -w myscript.pl "" # empty argument
perl -w myscript.pl 0 # zero argument
perl -w myscript.pl something-else # other argument