"May be with this you could help me out in a better way."

You'll need to specify what you want help with; an explanation of what you mean by "a better way" would also be useful. You should provide: some sample, representative input; actual and expected output (as well as an indication of where you're encountering difficulties getiing the actual output to match the expected output); any error and warning messages; along with anything else that helps to describe the problems you are encountering. All of this is explained in the guidelines "How do I post a question effectively?".

Here's some points on the code you've currently posted:

• You haven't used the strict or warnings pragmata. Unless you have a very good reason not to use these, you should include them in all your scripts. See the code I posted in my first reply to you.
• You've declared a number of lexical variables whose scope is the entire script but they're only used in Functionparser() — you should limit their scope to this subroutine.
• You should get into the habit of using the 3-argument form of open with a lexical filehandle. Using a package filehandle (such as FILE2 used here) can cause issues if the same name is used anywhere else in your script; a lexical filehandle (e.g. $fh) would have its scope limited to Functionparser() — another $fh used elsewhere in your script would be a completely different filehandle.
• Using $ARGV[0] in Functionparser() limits that subroutine. At some later time you may want to parse multiple files. Use something like "my ($source_code) = @_;" as the first line in that subroutine: it will then be more generalised and you can call it as many times as you want allowing multiple files to be parsed.
• Consider using the autodie pragma or using more descriptive error output than "die $!". The former only requires a single "use autodie;" near the start of your code; the latter is a lot more work and error-prone. I usually choose autodie unless I have a very good reason not to.
• You seem to understand what '$.' does (you've used "$func_start = $.") but you've also introduced $linenum to provide identical functionality (which requires additional processing, i.e. "$linenum++"). Given that '$.' is already available, it would be easier to just use that. Also note that you don't use $func_start after assigning a value to it.
• You have retained the original regex which was the basis of your OP question and that you said didn't work. Why? That regex contains two captures: you ignore the second (i.e. $2).
• Your use of next looks wrong. If the if block (with the next) is entered, the remaining code in the while loop will not be executed; if that if block is not entered, the remaining code in the while loop will still not be executed as all conditions checking $sw_test will be false. Those parts of the while loop may be executed on subsequent iterations; however, that looks rather flakey to me. Perhaps when you provide sample input, your intent may become clearer.

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