That's pretty nice... Here is a straight port from Ruby to Perl6:

sub evaluate ($expr) {
    my @stack;
    for $expr.split {
        @stack.push: do {
            when /^ -?\d* $/ { $_ }
            when <+ - * /> {
                my $right = @stack.pop;
                my $left  = @stack.pop;
                defined($left & $right) or fail "Stack underflow";
                $left.$_: $right;
            }
            fail "Invalid token: $_";
        }
    }
    @stack == 1 or fail "Invalid stack: @stack";
    @stack[0];
}
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