If only I had turned to page 166 in the book above I found a neat solution: Use caller to determine whether the script is called from the command-line (caller returns undef) or from a subroutine or "eval" or "require" (caller returns name of the calling package).
luckynumber.t

use warnings;
use strict;

use Test::More;
use Test::Exception;

BEGIN {
  plan tests => 1,
  todo => []
}

# ------ load script with arg - expect ok
ok( require 'luckynumber.pl', 'script loaded ok');
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luckynumber.pl

use strict;
use warnings;

use Getopt::Long;
my $luckynumber = undef;

sub main {
    GetOptions("luckynumber=i" => \$luckynumber) or die("GetOptions fa
+iled");
    die "missing argument luckynumber" unless defined($luckynumber);
    print "Lucky number is " . $luckynumber . "\n";
}

# See also perldoc -f caller
main() if! caller();

1;
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Running the test yields:

$ prove luckynumber.t
luckynumber....ok
All tests successful.
Files=1, Tests=1,  0 wallclock secs ( 0.06 cusr +  0.01 csys =  0.07 C
+PU)
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Andreas
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