$txtBox->tagRanges('sel'); returns begin/end coordinates of selection in <x,y> 2D format and if I extract data from widget via get() method it will be 1D string. Is there simple method to convert coordinates to 1D?

$txtBox->tagRanges('sel'); doesn't return 2D coordinates, it returns the line.column of the selected text start and end point(s). If you get() the text between those endpoints, you will have the selected text.

my @ranges = $txtBox->tagRanges('sel');
my $text   = $txtBox->get(@ranges[0,1]);
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When I select anything, I get error 'Tk::Error: wrong # args: should be ".frame1.text get index1 ?index2 ...?" at C:/Perl/lib/Tk.pm line 252'

Because you are not giving the get() method any indicies to work with. Try:

if(@r)
    {
        my $str = $txtBox->get(@r);
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Note that @r may have more than two coordinates. You may need to test and allow for them.

...if I use 'delete', 'insert' (and other Tk::Entry commands)...

Be aware that Tk::Entry methods != Tk::Text methods for the most part. There are many similarities, but many differences too. Do you want to do this with a Tk::Text box or a Tk::Entry? It would probably be easier with the Tk::Entry, it doesn't already have a defined action bound to the Return key. Tk::Text does, so you'll have to try to figure out what action the user meant when they hit Return. Not impossible, but a larger can of worms than I would want to open.