The array, @ARGV contains the various arguments passed on the command line. To access the first element by index, it would be $ARGV[0]. You're using that first element to handle the command line switch, L, U or D. The second arg is $ARGV[1], and that's where you're passing the key. The third (optional) parameter is $ARGV[2]. That element will either have a value, or it will be undefined. To detect which, you might do something like this:

if( defined $ARGV[2] ) {
    # Handle the search string.
}
else {
    # Perform whatever default preparations are necessary in
    # the absence of a defined search string.
}
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So the key is to test $ARGV[2] for definedness. There are a couple of useful ways to do that. The most common by far is the defined built-in function. But there's also the // and //= operators, described in perlop. With //=, you could do something like this:

$ARGV[2] //= 'Default value';
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In this case, if $ARGV[2] already contains a value, the line has no effect.  If it doesn't contain a value, '<c>Default value' is assigned.

Using the // operator might look like this:

my $pattern = $ARGV[2] // 'Default value';
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Here, $pattern receives the contents of $ARGV[2] if $ARGV[2] is defined. If it's undefined, then 'Default value' is assigned to $pattern.

The // and //= operators were introduced to Perl in version 5.10.0 (I think). So they won't work if you're stuck using older versions. defined works all the way back to the earliest Perl 5 versions, and possibly more.

Welcome to Perl! :)