Seems like a bug, or at least as an unintended use . Will try to fix, watch github for changes. Meanwhile, would that help you?

#!/usr/bin/perl --
use strict;
use warnings;

Main( @ARGV );
exit( 0 );

sub Main {
    ShowPrima();
    ShowPrima();
}
sub ShowPrima {
    require Prima ;
    require Prima::Buttons;
    require Prima::Application;
    $::application ||= Prima::Application-> create( qw' name Prima ');
+    
    my $mw = Prima::MainWindow->create(
        text     => 'Hello world!',
        size     => [ 200, 200],
    );
    $mw-> insert( Button =>
        centered => 1,
        text     => 'Hello world!',
        onClick  => sub { $mw-> cancel },
    );
    $mw-> execute;
}
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