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# run contents of "my_file" as a program
perl my_file
...
# command-line that prints the last 50 lines (expensively)
perl -e '@lines = <>; print @lines[ $#lines .. $#lines-50' f1 f2 f3 ..
+.

perl -000 -ne '$.=$a+1 and print "$.: doubled <$1> in\n$_\n" 
               if /\b(\w+)\b\s+\b\1\b/gi;$a+=tr/\n//' file