I am not sure exactly what you're asking. You wrote that $a and $b live in %main::, but in a sort routine, they actually are package globals in whatever package you currently happen to be in. Now, if you're stating that the variables that $a and $b are comparing live in %main:, that's irrelevant because they are aliasing the original variables.

If you want to use &directories_first as a sort routine, but it might be in a different package, you can use prototypes:

our %df_keys;
sub directories_first ($$) {
    $df_keys{$_[0]} ||= df_key($_[0]);
    $df_keys{$_[1]} ||= df_key($_[1]);
    return ( $df_keys{$_[0]} cmp $df_keys{$_[1]} );
}

# somewhere else in the galaxy:

my @new_array = sort Some::Package::directories_first @old_array;
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See perldoc -f sort for details. I hope this helps because I really didn't understand your question :)

Cheers,
Ovid

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package main;
our($a,$b);

package RealPackage;
# ...
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However, I think you are doing something that is not a good idea. Since you seem to have perl >=5.6, I would reccommend using a function with a prototype of ($$) as your sort routine:

sub directories_first ($$) {
   my ($a,$b) = @_;
   # ...
}
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Otherwise, if your function is to work should you want to use it in a package besides main you're going to need to do something to use the correct package's variable, such as dynamically generating the function on export or using symbolic references.

I notice that japhy says to use local and Ovid says to use my ... anyone have a preference and reason?

------
We are the carpenters and bricklayers of the Information Age.

Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.

Ovid is just using @_ directly. If the arguments are passed to the sub via local or my variables, it doesn't matter what they're called. The important thing is to use prototypes for the sort function. With that in mind, I think I'd still avoid local, and either use @_ directly or lexical/my variables.

    -- Chip Salzenberg, Free-Floating Agent of Chaos

And (just for fun) $main::main::main::a is a longer way to type </code>$main::a</code>

i.e. 'main' is somewhat like a symbolic link back to itself.....

-Blake