• Read perlintro.
• Write some code.
• Post back here with more detailed questions, if necessary.

A bit more info would be helpful. Read How do I Post a Question Effectively

or

My version of the same

Your problem statement breaks down as follows from my perspective:

• Get input from user
GetOpt::Long
• Open file for reading
open

If the file is not too large, you might consider reading it into and array(ex. my @lines)

• Search for line(?s) containing user provided characters
grep might work well for you
• Do calculation on lines found
maybe write a sub-routine to do your calculations?
• Provide output to user(file, screen, web page,...?) Might want to consider this

use strict;
use warnings;
etc...
[download]

Hi help_please
Welcome to Monastery.
You might want to try code some of the things you described, then others can help fill in areas where you don't get.
I think it is also great to check How do I post a question effectively?

Try this:

#!/usr/bin/env perl                                                   
+                         

use strict;
use warnings;

my $char = shift || die $!;
my $file = $0;

open( my $fh, "<", $file ) || die $!;

while (<$fh>) {
    print if /^o/; # your part goes here
}

close $fh;

__END__
[download]

Please see also strict, warnings, my, shift, die, while, open, print, if, close, perlretut as well as TMTOWTDI.

For the rest: RTFM ;-)

Update: Immediate feedback is a good thing. But please note: I really didn't want to be harsh or impolite. And i marked the dubious hint as a joke.

Regards, Karl

In addition to advice above re 'perlintro' and 'How do I ask a question,' please note:

Homework should be labeled as such!

And now you need to use Super Search to find out what else we expect of those asking homework questions.