sub extract {
   my ($filename, $line_no)=@_;
   my $line;
   open (FILE, $filename) || die "$filename can't be opened $! "
   if ($line_no =~ /\D/) {
        while ($line=<FILE>) {
            if ($line =~ /$line_no/) {
                return $line;
            }
        }
   }
   else {
     foreach (1..$line_no) {
           $line = <FILE>;
        }
        return $line;
   }
}

##</code><code>##

An example:
$file = "example.txt";
$line_no_reqd = 4; 
$result = &imp($file,$line_no_reqd);
print $result;