Working, tested code:

use strict;
use warnings;

my $usage_message = "usage: $0 input_file output_file\n";
die $usage_message if @ARGV != 2;

my ( $input_file, $output_file ) = @ARGV;

open my $in_fh,  '<', $input_file
  or die "Could not open input file 1 '$input_file': $!";
open my $out_fh, '>', $output_file 
  or die "Could not open output file '$output_file': $!";

while ( my $line = <$in_fh> ) {
    chomp $line;
    
    warn unless length($line) == 210;
    
    if ( $line =~ m/^2/ ) {
        # Do Nothing!
        # A line starting with a '2' is a
        # header and is left unchanged.
    }
    else {
        # Change postions 100 and 149
        substr( $line, 100-1, 1, '9' );
        substr( $line, 149-1, 1, 'Z' );
    }

    print {$out_fh} $line, "\n";
}

close $in_fh
  or warn "Could not close input file 1 '$input_file': $!";
close $out_fh
  or warn "Could not close output file '$output_file': $!";
[download]

perl -wlpe 'if(!/^2/){substr($_,99,1,"9");substr($_,148,1,"Z");}' <in.
+dat >out.dat
[download]

Util,

Thanks so much for the code. As a Newbie this has taught me a lot and I am astounded at the concise code to accomplish what originally started with 100 lines or so on my first convoluted attempt and now it can be distilled into one line.

Thanks to you and to the other contributers,

Stephen

        $output_line = substr( $line, 0, 98 ) 
        . $juris1 . substr( $line, 100, 48 )
        . $juris2 . substr( $line, 149, 61 ) 
        . "\n";
[download]

can be made a bit more readable:

        $output_line = "$line\n";
        substr($output_line,  99, 1) = $juris1;
        substr($output_line, 148, 1) = $juris2;
[download]

or

        $output_line = "$line\n";
        substr($output_line,  99, 1, $juris1);
        substr($output_line, 148, 1, $juris2);
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Otherwise, it looks like the only problem you have is that you're not printing out the lines starting with "2", and you end the loop prematurly using last.

    while (<INFILE>) {
        my $line = $_;
        if ( $line !~ m/^2/ ) {
            substr($line,  99, 1, "9");
            substr($line, 148, 1, "Z");
        }
        print OUTPUT $line;
    }
[download]

Are you having any other problems?

Or if you're trying to preserve all the lines starting with the line starting with "2", then:

    my $found_2;
    while (<INFILE>) {
        my $line = $_;
        if ( !$found_2 ) {
            if ( $line ~= m/^2/ ) {
                $found_2 = 1;
            } else {
                substr($line,  99, 1, "9");
                substr($line, 148, 1, "Z");
            }
        }
        print OUTPUT $line;
    }
[download]

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

                # -----A line starting with a '2' is a header and is l
+eft 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;
                }
        }
[download]

that there's no 'else' for your 'if'. What are you supposed to do when you hit /^2/, again? if you've found the /^2/ line, just stream through the rest of the file?

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

                # -----A line starting with a '2' is a header and is l
+eft 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>;
                }
        }
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But your negated 'if' condition and the comment above it confuses me a bit. Shouldn't that be

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

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

while (<INFILE>) {
    my $line = $_;
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with foreach? If that is the case, there's no need for foreach, just replace those two lines with:

while (my $line = <INFILE>) {
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Did you know that substr can take a 4th argument?

$juris1 = substr( $line, 99, 1 );
$juris2 = substr( $line, 148, 1 );

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

#or
#
#substr( $line, 99, 1, '9' ); # 4th argument replaces
#substr( $line, 148, 1, 'Z' );# selected sub-string
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It doesn't look like you're using strict and warnings. They are real timesavers and you should look in to using them.

perl -pi.bak -e '/^2/ or substr $_, 99, 1, 9 and substr $_, 148, 1, "Z
+"' input_file_name
[download]