Consider the following snippet:

sub account_over_limit {
    my ($account, $balance) = @_;
    return exists $LIMITED_ACCOUNTS{$account} && $balance > $LIMIT;
}
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You might think that since the %LIMITED_ACCOUNT and $LIMIT variables are relatively static, declaring them as globals is a good idea. In reality, this makes life very difficult if you need to refactor the code or track down bugs. Obviously, you now have a routine that cannot be cut and pasted into another package as part of a refactoring strategy because you're dependent on external data. This is a big problem.

Next, consider what happens when you realize that different accounts have different limits. Your code changes $LIMIT at one point and forgets to reset it. Now your &account_over_limit subroutine returns spurious results, but this problem might not show up for weeks until your accounting department realizes that some people have received more credit than they should, or are complaining when they're not allowed to buy something.