I found nothing about that in the sort documentation, like you I imagine. I only guess that this check for the prototype is done at run-time instead of compile time, as the following modification may suggest:

#!/usr/bin/perl -w
use strict;

package joe;
my @names = ('joe', 'aaron');

package FP;
sub sortNames ($$)
{
    print "$_[ 0 ] vs $_[ 1 ]\n";
    return $_[ 0 ] cmp $_[ 1 ];
}

package joe;
my @sorted = sort nonexistent @names;
print "sorted : @sorted\n";

CHECK {print "here I am\n"}

__END__
here I am
Undefined subroutine in sort at asort.pl line 15.
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