The only output to a file you have here is:

               $dst ||= "$0.edit";  #in case user hits cancel 
               warn "dst=$dst";

         open (my $fh, '>',$dst) or die "$!\n";
         print $fh $sublable->get("1.0","end");
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Your original file is only read:

   my $fh;
   open( $fh, '+<', "./test.txt" ) or die $!;
   my @contents = <$fh>;
#   print "@contents\n";
   close( $fh );
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Meaningful variable names would probably help you.

Update:

There may also be a problem here:

            my $dst  = $mw->getSaveFile(
            -initialdir       => $ENV{HOME},
            -defaultextension => '.in',
             -title            => 'Save',
            -filetypes        => [
                [ 'myfiles'    => '.in' ],
                [ 'All files' => '*'    ],
            ],
            );
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Please post the output from your code. My problems in determining what's going wrong here stem from:

• Opening the original file (./test.txt) in read-write (+<) mode but then only reading it.
• Whether you're experiencing some confusion with using my $fh twice.
• Your original file is in the current directory (./) but you're specifying -initialdir       => $ENV{HOME}. What's the full pathnames of these?
• The extension of your original file is .txt but you're specifying -defaultextension => '.in'
• Your zig-zag indentation style doesn't help much either.

Thanks for giving replay, here is my updated code , my input file "./test.txt" (original file) consists 20-30 lines of data , if i will open the same file through my script it will show very few lines and it is not saving any changes into original file.

#!/usr/bin/perl -w
use warnings;
use strict;
use Tk;
use Tk::FileSelect;

my $mw = MainWindow->new;
$mw->configure( -background => 'black', -foreground => 'white' );
$mw->geometry( "400x300" );
$mw->title( "Multiple Windows Test" );
my $curr_path = "/root/test.txt";

my $button1 = $mw->Button(
   -text       => "view Results",
   -background => "cyan",
   -command    => \&button1_sub
)->pack( -side => "right" );

$mw->Button( -text => "Exit", -command => sub { exit } )
 ->pack( -side => "bottom" );

sub button1_sub {
        my $subwin1 = $mw->Toplevel;
        $subwin1->geometry( "500x400" );
$subwin1->title( "Sub Window #1" );

        my $fh;
        open( $fh, '+<', "$curr_path" ) or die $!;
        my @contents = <$fh>;
        close( $fh );

        my $sublable = $subwin1->Scrolled( 'Text', -scrollbars => 'oso
+e', )->pack;
        $sublable->insert( 'end', @contents );

        my $subwin_button = $subwin1->Button(
                                -text    => "Close window",
                                -command => [$subwin1 => 'destroy'],
                                )->pack( -side => "bottom" );

#=================Creating save buttion on subwindow ===========

my $save_button = $subwin1->Button(-text=>'save',
                                -command =>\&get_save,
                                -background =>'cyan'
                                )->pack(-side=>'right');
}
MainLoop;
sub get_save {

        my $dst  = $mw->getSaveFile(
            -initialdir       => '/root/',
            -defaultextension => '.txt',
            -initialfile      =>'test.txt',
            -title            => 'Save',
            -filetypes        => [
                [ 'myfiles'   => '.txt' ],
                [ 'All files' => '*'    ],
            ],
        );

      $dst ||= '<undef>';
      warn "dst=$dst";
}
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</code>

I asked: "Please post the output from your code.". Is there a reason you can't or won't do that?

-- Ken