while (<INFILE>) {
                my $line = $_;
                chomp($line);

                # -----A line starting with a '2' is a header and is left unchanged
                if ( $line !~ m/^2/ ) {

                        # -----Locate postions 100 and 149
                        $juris1 = substr( $line, 99, 1 );
                        $juris2 = substr( $line, 148, 1 );

                        $juris1 = "9";
                        $juris2 = "Z";

                        # -----Generate the output string-----
                        $output_line = substr( $line, 0, 98 )
                        . $juris1 . substr( $line, 100, 48 )
                        . $juris2 . substr( $line, 149, 61 )
                        . "\n";

                        print OUTPUT $output_line;
                        last;
                }
        }

##</code><code>##

        while (<INFILE>) {
                my $line = $_;
                chomp($line);

                # -----A line starting with a '2' is a header and is left unchanged
                if ( $line !~ m/^2/ ) {

                        # -----Locate postions 100 and 149
                        $juris1 = substr( $line, 99, 1 );
                        $juris2 = substr( $line, 148, 1 );

                        $juris1 = "9";
                        $juris2 = "Z";

                        # -----Generate the output string-----
                        $output_line = substr( $line, 0, 98 )
                        . $juris1 . substr( $line, 100, 48 )
                        . $juris2 . substr( $line, 149, 61 )
                        . "\n";

                        print OUTPUT $output_line;
        #                last;  # <-- this last exits the input loop, no more lines read.
                                # Is that really what you want to do?
                                # exit the loop after the first non - /^2/ - line?
                } else {
                        print OUTPUT while <INFILE>;
                }
        }


##</code><code>##

                # ----- Do as long as line doesn't start with '2'