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##  Perl Helpers - Useful for setting up Perl environment
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(Perl with a "-e" allows Perl to work on command line)
perl -e "print \"@INC\""

TO USE CPAN on RedHat family linux:
yum install bind-utils
yum install perl perl-devel gcc curl
dnf install perl-CPAN
yum install expect
yum install bind-utils

To get Expect from CPAN:
cpan App::cpanminus
cpanm Term::ReadKey
cpanm Net::SSH::Expect

COMMAND LINE PERL
CHANGING A STRING in MULTIPLE TEXT FILES IN DIRECTORY
For all files in this directory, change internal variable $REPLACEMENT to $REPLACEMENT
ALSO rename the original files to <filename>.original

perl -i.original -pe 's/\$REPLACEMENT/\$REPLACEMENT/g' *

Same as above but without retaining the original files:

perl -i -pe 's/\$REPLACEMENT/\$REPLACMENT/g' *

The -p means to enclose your Perl program in a print loop that looks SOMEWHAT like this:

while ($_ = <>) {
    <Your Perl one liner>
    print "$_";
}

SPECIAL CASE:  remove hat-em (^M) and replace it with regular carriage return:
perl -i -pe 's/^M/\n/g' filename
or for all files in current directory
perl -i -pe 's/^M/\n/g' *

* to create the "hat-em" you need to press Ctrl-v and then Ctrl-m  or Ctrl-v, m

And while we are at it, this sed command on command line will remove null charactors from files:

sed -i 's/\x0//g' filename.txt

Remember to diff your original and replacement files to make sure the outcome is what you expected....

OOOOOPPPPPSSS!!!!!
What do you do if you made a mistake?

simple

mkdir REVERT_ME
cp -a *.original REVERT_ME
cd REVERT_ME

# rename files from <filename>.original to <filename>
perl -e 'while(<*.original>){$_=~/(.*)\.original/; rename$_,$1}'

Now, just replace the files in the original subdirectory with the files
in the REVERT_ME subdirectory.