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use strict;
...
# finally, hardlink the files in each group:
reduce { system qq( ln "$a" "$b" ); $b } @$_ for @a;

map { reduce { system qq( ln "$a" "$b" ); $b } @$_ }

...
partition_by_calculated_key { -s $_ } 

[ grep { ! -l $_ } grep { ! -d $_ } @files ]