BOL

Nice collection of Perl oneliner. Thank you for the useful list. You can also track other perl onliner posted on wire at http://bioinformaticsonline.com/thewire/tag/Oneliner

Convert file to lowercase

dd 
$ dd if=input.txt of=output.txt conv=lcase

tr
$ tr '[:upper:]' '[:lower:]' < input.txt > output.txt

awk
$ awk '{ print tolower($0) }' input.txt > output.txt

perl
$ perl -pe '$_= lc($_)' input.txt > output.txt

sed
$ sed -e 's/\(.*\)/\L\1/' input.txt > output.txt

This tutorial is useful https://blogs.oracle.com/linux/the-top-10-tricks-of-perl-one-liners-v2