Now as you can see in the comment in your question that you really need to be specific about where you are actually stuck in to ask a question in SO. However, after looking at your other questions in SO it looks like you are completely new in programming world and need serious helping hand. Hence I think it would be helpful for you to get some useful reference to solve your problem.
If I breakdown your main problem statement I can see three parts
1. Need to extract the file name,
Most recommended way to do it is using File::Basename. However, its possible to use regex too. You need to learn how to write regex and how to capture a group. This link should be really helpful.
2. Compare it with variable,
Whichever method you use from above you should get the filename in a variable or in $1. Just compare that whatever you want to compare with. Make sure to use eq instead of ==. Find more details here.
3. Append the new variable to the string,
By now you should have the old filename in a variable, say $oldname, from first step and the new filename in another variable, say $newfilename. This question is already answered several times in SO, like this one. Hope that helps.
The actual solution is merely a 3 or 4 lines of code which I want you to figure out. Good luck.
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solved String replacement in Perl after comparing with the original string