Here's another explanation:

#!/bin/bash

# enable debug
set +xe 

# this is an old school way to check for an empty variable    
# modern shells can use [[ $foo == "" ]] to do this
if [ x"$Link_A" == "x" ] ; then

    # a normal error message for when the variable is not set
    echo "Link_A parameter not set, DOESNOT execute FUNCTION"
else # the variable has a value so we can use it...

    # copy this file to the current directory
    cp -f ~/cached-resources/www.github.com/file1/executables/JenkinsScripts/jenkins_feed.pl ./jenkins_feed.pl

    # run the script in perl with arguments that include variable
    # substitution and some fixed text
    perl jenkins_feed.pl $JENKINS_HOME/feed_bts.ini $Link_A
fi

I also reformatted the script so the inner parts of the conditional are indented.

bonus questions from comments

1. Is Link_A a file ?

Link-A is a variable. A variable could point to a file or directory, but it isn't clear what the variable would contain in this case. Examining the perl script would be necessary to answer this definitively.

2. what does feed_bts.ini means do i need to create a file like this explicitly if not present?

I'd guess the perl script is expecting a file name as its first argument. The shell script provides this and it expects the feed_bts.ini file to be in the $JENKINS_HOME (another variable) directory.

3. Do I need to keep the jenkins_feed.pl and feed_bts.ini in same directory to run the script?

The jenkins_feed.pl ends up in the same directory as the shell script because of the cp. It starts as ~/cached-resources/www.github.com/file1/executables/JenkinsScripts/jenkins_feed.pl. Note that ~ would the home directory of the user the script runs as.

4. from which path is the perl script executing

This would probably be using whatever the stock perl installed with your distro is. Technically the shell will look through all of the directories in your $PATH until it finds it or runs out of directories.

perl script analysis

• The second argument $Link_A has three pieces of information in it. On https://github.com/rebeccaus/perl/blob/master/feed.pl#L17 this gets split into three variables: $Link_A,$skey,$priority.

  1. The first part of this gets stuffed back into the same-named variable which only gets used at https://github.com/rebeccaus/perl/blob/master/feed.pl#L90. That looks like it is just arbitrary text that shows up in a particular place in the description ($desc) which ends up in the Jenkins XML field IV_DESCRIPTION.
  2. The $skey goes into the Jenkins XML field IV_EVENT_KEY. There's a default value for this just in case at https://github.com/rebeccaus/perl/blob/master/feed.pl#L129.
  3. The $priority goes into the Jenkins XML field IV_PRIORITY.

• I didn't find anything convenient to define what those fields are doing. They are part of a SOAP call to "feed_issue_CREATION".

• The first argument to the script looks like a config file.

  1. https://github.com/rebeccaus/perl/blob/master/feed.pl#L14 reads the first argument from the command line. If it is missing you go to the usage subroutine.
  2. Then https://github.com/rebeccaus/perl/blob/master/feed.pl#L19 checks to make sure it exists and it is a file. If not go to the usage subroutine again.
  3. https://github.com/rebeccaus/perl/blob/master/feed.pl#L49 calls the readopeninc subroutine with the config file name as the argument.
  4. https://github.com/rebeccaus/perl/blob/master/feed.pl#L287 defines the readopeninc subroutine which seems to do a few things. It ignores lines that start with a # as the absolute first character. It strips any leading and trailing white space. Then it assumes you're got variable assignments like the shell: VAR_NAME=SOME_VALUE
  5. After that the it seems to use the values in there as jobs that need to be updated or closed.
• based on https://github.com/rebeccaus/perl/blob/master/feed.pl#L115 it looks like the $inclog aka feed_INC.txt is in the same format somehow.
• based on https://github.com/rebeccaus/perl/blob/master/feed.pl#L80 it is looking for the files to be in the current directory.