Your code is very convoluted, and what you should do, is make the initial frame separate from the added frame, but I'm not rewriting your code. That way you can just gridForget the entire popup frame. As it is, you need to get all the gridSlaves of the Frame, pop off the widgets you need to save, and rebuild.

But since this isn't a code writing service, here is a convoluted way to fix your convoluted code. :-) Look at the sub do_reset for what is going on, and read perldoc Tk::grid.

This is not the best way, but it works.

#!/usr/bin/perl
use warnings;
use strict;
use Tk;
use Cwd;

######################################################################
+#####################
# GUI Building 
######################################################################
+#####################

# make the selections available to the print_input_values sub
my %t;
my %mat;
my %ort;
my %mat_lab;


# Create Main Window
my $mw = new MainWindow;
my $lam_num;
my $row=1;
my $column=1;

my $lam_mat_frm = $mw->Frame();

$lam_mat_frm->grid(-row=>$row, -column=>$column,-columnspan=>6);

my $lam_num_lab = $lam_mat_frm -> Label(-text=>"Input the number of pl
+ies in the laminate.",
    -font=>"ansi 10 bold");
$lam_num_lab->grid(-row=>$row, -column=>$column);

$column++;
my $lam_num_ent = $lam_mat_frm -> Entry(-textvariable=> \$lam_num);
$lam_num_ent->grid(-row=>$row, -column=>$column);


$column++;
my $lam_data_button = $lam_mat_frm->Button(-text=>"Input Laminate Data
+",
    -command=> \&input_lam_data);
$lam_data_button->grid(-row=>$row, -column=>$column);

$column++;
my $reset_button = $lam_mat_frm->Button(-text=>"Reset",
    -command=> \&do_reset);
$reset_button->grid(-row=>$row, -column=>$column);


MainLoop;


sub do_reset{ 

$lam_num_ent->delete(0,'end');

my @slaves = $lam_mat_frm->gridSlaves;
print "@slaves\n";

pop @slaves for (1..4); # save the first 4 widgets
print "@slaves\n";

foreach my $slave( @slaves){
    $slave->gridForget;
    print "$slave\n";
  }

}


sub input_lam_data {
    $row=4;
    $column=2;
    my @lam_mat_t_lab = (
    'Material',
    'Thickness',
    'Orientation',
    );
    my $n = 1;
    for my $label (@lam_mat_t_lab) {
        $mat_lab{$n} = $lam_mat_frm -> Label (-text=> $label,
        -font=>"ansi 8 bold");
        $mat_lab{$n}->grid(-row=>$row, -column=>$column);
        $column=$column+1;
    };
    my %ply_lab;
    $n = 1;
    $row=5;
    do {
        $column = 1;
        $ply_lab{$n} = $lam_mat_frm -> Label (-text=>"Ply $n",
        -font=>"ansi 8 bold");
        $ply_lab{$n}->grid(-row=>$row,-column=>$column);
        $row = $row+1;
        $n = $n+1;
    } until ($n == $lam_num+1);

    # Create Material Optionmenus
    my @mats = ("PW","8HS","Tape");
    my %mat_optmen;
    $n = 1;
    $row=5;
    $column=2;
    do {
        $mat_optmen{$n} = $lam_mat_frm -> Optionmenu(-options => \@mat
+s,
        -variable => \$mat{$n});
        $mat_optmen{$n}->grid(-row=>$row,-column=>$column);
        $row=$row+1;
        $n = $n + 1;
    } until ($n == $lam_num+1);
    
    # Create Thickness Optionmenus
    my @ts = ("0.0077","0.0147","0.0054");
    my %t_optmen;
    $n = 1;
    $row=5;
    $column=3;
    do {
        $t_optmen{$n} = $lam_mat_frm -> Optionmenu(-options => \@ts,
        -variable => \$t{$n});
        $t_optmen{$n}->grid(-row=>$row,-column=>$column);
        $row=$row+1;
        $n = $n + 1;
    } until ($n == $lam_num+1);

    # Create Orientation Optionmenus
    my @orts = ("-45","0","45","90");
    my %ort_optmen;
    $n = 1;
    $row=5;
    $column=4;
    do {
        $ort_optmen{$n} = $lam_mat_frm -> Optionmenu(-options => \@ort
+s,
        -variable => \$ort{$n});
        $ort_optmen{$n}->grid(-row=>$row,-column=>$column);
        $row=$row+1;
        $n = $n + 1;
    } until ($n == $lam_num+1);
    my $print_button = $lam_mat_frm->Button(-text=>"Print BJSFM Input 
+File",
    -command=> \&print_input_file);
    $print_button->grid(-row=>$row,-column=>1);
}
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