1. Load a module that is never used.

#!/usr/bin/perl
use strict;

use Foo;
use Bar;                    # Needless

my $foo = Foo->new();
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2. Forget to explicitly load a module.

#!/usr/bin/perl
use strict;

use Foo;

my $bar = Bar->new();       # Works because Bar is loaded by Foo
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In a separate module...

package Foo;

use Bar;

sub new {
    ...
}
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If package Foo is ever modified such that it no longer uses Bar, the script goes kaboom.

Any suggestions on how to reveal these in scripts other than scanning the code? Do you use any practices to prevent them?