Suppose you want to write and test a function that adds number, you could do it like this:

use strict;
use warnings;

sub add { $_[0] + $_[1] };

if (defined $ARGV[0] && $ARGV[0] eq 'test') {
    use Test::More;
    plan tests => 1;
    is add(1, 2), 3, 'Can add two numbers';
} else {
    print add(@ARGV), "\n";
}
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And run it as

$ perl foo.pl test
1..1
ok 1 - Can add two numbers
$ perl foo.pl 4 6
10
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Ok, the example is really lame, but it shows how you can approach the whole thing.