Tutorial

Command line flags and parameters

For the official documentation visit perldoc perlrun or the same on MetaCPAN

-v print the version of perl
--version the same as -v
-V print defailed information about the perl. (e.g. compilation flags)
-e Execute the perl code that follows this flag.
-E Just like -e, but enable all extra features add to Perl since the 5.10 release.
-n Wrap the code provided using the -e or -E flags in a while loop to execute the code on every line of every file. The current line is in $_.
-p Do the same as -n and also print the possibly modified content of the current line stored in $_.
-i in-place editing (replace the processed file by the output)
-i.bak in-place editing with backup

Regex (or regexp or Regular Expression) /bla/ or m/bla/ or m{bla} where bla can be any regular expression. See perlre or on MetaCPAN.
Substitution with regex s/bla/replacement/.
if - conditional
not - boolean negation
unless the same as if not
$. the current line number starting from 1

Use version control (e.g. git) on all of your file, including your data files.
When doing in-place editing, if you don’t have version control of the files you are editing then create a backup either before the process or during the process using -i.bak
Quotes: On Linux and macOS we usually use single-quotes around the perl code of the oneliner. On MS Windows AFAIK you cannot do that and thus there the outer quotes are double-quotes. This also means that the quotes, if used in the code, will need to be used differently. Therefore in general it is better to use q() instead of single quotes and qq() for double quotes.