It doesn't have any special meaning per se, but by convention the arguments passed to a module are normally a list of subroutines that you wish to import into your namespace. Thus if a module wants to allow you to specify other options, it makes things clearer if those arguments don't "look like" subroutine names. So you often see such options starting with characters that are not allowed in subroutine names, such as '-', '!', ':' or '+'.

The minus sign has an added advantage over other characters that due to fairly convoluted Perl parsing rules, the following:

use Foo "-bar";
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... can be written as:

use Foo -bar;
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... without violating use strict. In fact, you can even write it as:

use Foo-bar;
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