Please put at least some effort into it!

Hint: Read the (key,value) pairs from the file into a hash. Make the changes and write it back to the file. The following snippet from the Perl cookbook, I used many times to read config files into a hash.

while (<CONFIG>) {
    chomp; # no newline
    s/#.*//; # no comments
    s/^\s+//; # no leading white
    s/\s+$//; # no trailing white
    next unless length; # anything left?
    my ($var, $value) = split(/\s*=\s*/, $_, 2);
    $User_Preferences{$var} = $value;
}
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Cheers

Harry

Update

Misunderstood what you want. Take a look at Term::ReadKey.

So, what code have you written, and what problems did you encounter when writing it? Commonly, the modules for reading a configuration file are also able to write a similar config file.