The problem is caused by the fact that you are binding <Key-Escape> to sub { exit }. If you lookup exit in perlfunc, you find it performs the syscall to shutdown your whole program. What you need instead is to have a closure on $x which call's the relevant function to destroy the MainWindow.

for (@ARGV) {
        my $x = MainWindow->new;
        $x->bind("<Key-Escape>", sub { $x->destroy });
}
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