Cannot modify readonly value in '&infix:<=>'

gistfile1.txt

sub make_flipflop($/) {

    my $excl_lhs  := 0;
    my $excl_rhs  := 0;
    my $sedlike   := 0;
    if ($/<infix><sym> eq '^ff' || $/<infix><sym> eq '^ff^'
            || $/<infix><sym> eq '^fff' || $/<infix><sym> eq '^fff^') {
        $excl_lhs := 1;
    }
    if ($/<infix><sym> eq 'ff^' || $/<infix><sym> eq '^ff^'
            || $/<infix><sym> eq 'fff^' || $/<infix><sym> eq '^fff^') {
        $excl_rhs := 1;
    }
    if ($/<infix><sym> eq 'fff' || $/<infix><sym> eq '^fff'
            || $/<infix><sym> eq 'fff^' || $/<infix><sym> eq '^fff^') {
        $sedlike := 1;
    }


    my $lhspast   := $/[0].ast;
    my $rhspast   := $/[1].ast;
    my $state_var := $lhspast.unique('ff_state');

    # define state-saving pir
    my $label := $lhspast.unique;
    my $get_state := "    %r = get_hll_global ['GLOBAL'], '$state_var'
    unless null %r goto ff_state_assign_$label
    new %r, 'Integer'
    assign %r, 0
    set_hll_global ['GLOBAL'], '$state_var', %r
  ff_state_assign_$label:
    %0 = %r";

    my $set_state := "    set_hll_global ['GLOBAL'], '$state_var', %0";


    my $lhsinc := ($excl_lhs) ?? '&postfix:<++>' !! '&prefix:<++>';
    my $rhsinc := ($excl_rhs) ?? '&postfix:<++>' !! '&prefix:<++>';
    my $past := PAST::Op.new(
        :pasttype<stmts>,
        PAST::Op.new(
            :pasttype('inline'),
            :inline($get_state), # state_var exists
            PAST::Var.new( :name($state_var), :scope('lexical'), :isdecl(1), :viviself(0) ),
        ),
        PAST::Op.new(:pasttype<unless>,
            PAST::Var.new(:name($state_var), :scope<lexical>),
            PAST::Op.new(:pasttype<if>,
                $lhspast,
                PAST::Op.new(:name($lhsinc),
                    PAST::Var.new(:name($state_var), :scope<lexical>)
                )
            )
        ),
        PAST::Op.new(:pasttype<if>,
            PAST::Var.new(:name($state_var), :scope<lexical>),
            PAST::Op.new(:pasttype<if>,
                $rhspast,
                PAST::Op.new(:name('&infix:<=>'),
                    PAST::Var.new(:name($state_var), :scope<lexical>),
                    PAST::Val.new( :value(0) )),
                    # $*ST.add_constant('Int', 'int', 0)),
                PAST::Op.new(:name($rhsinc),
                    PAST::Var.new(:name($state_var), :scope<lexical>))
            )
        ),
#        PAST::Op.new( :inline($set_state), PAST::Var.new( :name($state_var), :scope('lexical') ) ),
    );
    if $sedlike {
        $past.pasttype('unless');
        $past.unshift(PAST::Var.new(:name($state_var), :scope<lexical>));
    }

    $past := PAST::Op.new(
        :pasttype<unless>,
        $past,
        PAST::Val.new( :value('') ),
        # $*ST.add_constant('Str', 'str', '')
    );
}