Main problem is that you never call MainLoop().

The following code does what you seem to want without the need for the $ep variable (and I didn't include the $TEST stuff because it's not relevant to the problem that I can see):

use Tk;

my $PATH = "/opt/adacs/test/$TEST/.saved_poins";
open( RF, $PATH ) or do_err_window() && exit;

print "This got executed\n";

sub do_err_window    
{
    my $fail = MainWindow->new();
    $fail->geometry("250x150");
    $fail->Label(-text => "File Failed To Open!")->
       grid( 
        $fail->Button(-text => "Acknowledge", 
                  - command => sub { exit}
                  )
        ); 
    MainLoop();
}
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