One problem that I am getting is , the font is always set to Helvetica and size as 9.

That happens when you close and restart the application, right? The "old font" is the default font, in this case probably Helvetica,9. If you restart the application the default font will be used. Use a configuration file to save the desired font and apply it at the application start.

The same for the HList, if you destroy and recreate it you have to reapply the desired font.

Tk::Style might help with that (I havent tried it)

Yes,this is certainly happening when I restart the application. Even when my gui is getting refreshed by $mw->after() every 10 secs , the selected font is also getting lost from the itemstyles.
Please let me know how can I apply the desired font on HList effectively, I am applying it in the callback on the entire mainwindow widget as $mw->RefontTree(-font => $font) , is not that enough ?

It's not enough because you don't call RefontTree again after destroing the HList. ItemStyle has a font option, make $font global and use it to set the font when the HList is recreated (-font => $font).

In reply to your comment
"It's not enough because you don't call RefontTree again after destroing the HList. ItemStyle has a font option, make $font global and use it to set the font when the HList is recreated (-font => $font). "
While seleceting the fonts from the FontDialog, I was also printing the $font and its descripttive name. For example for 'courier 10 pinch' and font size 5 , the $font which we pass to -font is coming as font30.
I created a global variable at the top of my program as $font and assigned font30 to it. Then using that $font to RefontTree for $mw and $hlistframe. Even after destroying the hlistframes i am calling the RefontTree on $hlistframe and passing $font to -font. But it does not seem to be taking that font while the gui is launched.
Can you please show me a working example.

It seems that despite what the docs say, you have to use the descriptive name to set the font.

Anyhow a better way than destroying and recreating the HList widget, is to clear the contents and than add new contents like in the following example:

use strict;
use warnings;

use Tk;
use Tk::HList;
use Tk::ItemStyle;
use Tk::FontDialog;

my $mw = MainWindow->new;

$mw->title("View");
$mw->minsize(560, 180);

my $font = '{Architects Daughter} -12 bold'; # have to use descriptive
+ name

my $top
    = $mw->Frame()->pack( -side => 'top', -fill => 'both', -expand => 
+0 );
my $hlistframe
    = $mw->Frame()->pack( -side => 'top', -fill => 'both', -expand => 
+1 );
my $bottomframe = $mw->Frame()->pack( -fill => 'x', -expand => 0 );

my $menubutton
    = $top->Menubutton( -text => "Font", )->pack( -side => 'left' );
$menubutton->command(
    -label   => "Change",
    -command => sub {
        $font = $mw->FontDialog->Show;
        $mw->RefontTree( -font => $font ) if defined $font;
        print "Font changed: $font\n";
    }
);

my $hl = $hlistframe->Scrolled(
    'HList',
    -scrollbars => 'ose',
    -columns    => 2,
    -header     => 1,
    -height     => -1,
)->pack( -fill => 'both', -expand => 1 );

my $label1 = $hl->Label( -text => "Tool", -anchor => 'w' );
$hl->headerCreate( 0, -itemtype => 'window', -widget => $label1 );
my $label2 = $hl->Label( -text => "Vendor", -anchor => 'w' );
$hl->headerCreate( 1, -itemtype => 'window', -widget => $label2 );
#...

$bottomframe->Button( -text => "Add item", -command => \&add_item )
    ->pack( -side => 'right', -anchor => 'ne', -fill => 'x' );

$bottomframe->Button( -text => "Clear", -command => \&clear )
    ->pack( -side => 'right', -anchor => 'ne', -fill => 'x' );

$mw->RefontTree( -font => $font ) if defined $font; # set default font

my $style = $hl->ItemStyle(
    'text',
    -background => 'light green',
    -font       => $font,
);

add_item();

sub add_item {
    my $e = $hl->addchild('');
    if (defined $font) {
        my $font_descriptive = $mw->GetDescriptiveFontName($font);
        $style->configure( -font => $font );
        print $font_descriptive, "\n";
    }
    $hl->itemCreate( $e, 0, -text => 'Col0', -style => $style );
    $hl->itemCreate( $e, 1, -text => 'Col1' );
}

sub clear {
    $hl->delete('all');
}

MainLoop;
[download]

This way changing the font of the widget is not needed anymore.

Thanks for your reply.
The fonts are working.
But the frame packing problem has surfaced again. I made the following 3 changes you mentioned
1.Minsize for main window
2.top and bottom frame expand=>0 and fill=>x
3.height=>-1 to HList
Doing the above things, frames are getting packed as I want and the 'Display' button on bottomframe is visible at the minimum size of the window. But this works only if I have 2-3 rows of data under the columns.

If I have some 6+ rows , then the gui window is launched with a bigger size and as I contract it to a smaller size the Display button on the bottomframe starts disappearing. Hence the problem still persists. Also the vertical scrollbar on the right hand side has no use if the gui always has to be kept so long to view some 6+ rows. It should stay as small as (380x160) irrespective of the row numbers and one can make use of the scrollbar and yet see all the frames.

One more thing : Clicking on the 'Display file' button opens a scrolled ROText widget. Hence the default font or the selected font if I choose any from the FontDialog later, is getting applied on the ROText widget as well. I think since I am applying RefontTree on the mainwindow ($mw->RefontTree) so it is happening. But can I prevent that anyway ? By that I mean, on the ROText I want the normal Tk font that happens if I do not hard-code any default font or so by RefontTree .
To consolidate, the default font or the selected font from FontDialog will still be applied on $mw but not on the ROText window.