I had to modify a file to remove duplicate lines and be sorted by length of line, so I used the following one-liner: 
perl -plwe'$h{$_}++ } for (sort { length($a)<=>length($b) } keys %h) {
+' infile >infile.tmp

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and then moved the infile.tmp to infile. My task was accomplished, but I couldn't help but wonder if it could have been easily accomplished with inplace editing. I hadn't tried at first, because I thought the print would no longer go to the correct output file once the input file was completely read; a quick try confirmed this: when the implicit readline is returning undef, it also closes ARGVOUT and re-selects STDOUT. After a few seconds, the solution became obvious: 
perl -i.bak -plwe'$h{$_}++; last if eof } for (sort { length($a)<=>len
+gth($b) } keys %h) {' infile

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In reply to Using -i in oneliner with postprocessing by Anomynous Monk

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