You can avoid exporting $a and $b by defining the sort routine like this: 
sub caseless ($$) { lc $_[0] cmp lc $_[1] }

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though the docs for sort say this is slower than using the un-prototyped version with $a and $b.

In reply to Re: sort routines crossing package lines by Arunbear
in thread sort routines crossing package lines by DrWhy

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