# Mostly extracted from the documentation.  I have not tried it.

use File::MergeSort;

my $sort = new File::MergeSort( 
   \@file_list, 
   \&index_extract_function 
);

my $line;
while (defined($line = $sort->next_line)) {
   my $fname = getFilename();
   print "$line/$fname\n";
}