RPN to infix conversion

sub p ($pair, $prec) { $pair.key < $prec ?? "( {$pair.value} )" !! $pair.value }

sub rpm-to-infix($string) {
    my @stack;
    for $string.words {
        when /\d/      { @stack.push: 9 => $_ }
        my ($y,$x) = @stack.pop, @stack.pop;
        when '^'       { @stack.push: 4 => ~(p($x,5), $_, p($y,4)) }
        when '*' | '/' { @stack.push: 3 => ~(p($x,3), $_, p($y,3)) }
        when '+' | '-' { @stack.push: 2 => ~(p($x,2), $_, p($y,2)) }
    }
    ($string, @stack».value).join("\n") ~ "\n";
}

say rpm-to-infix $_ for
    '3 4 2 * 1 5 - 2 3 ^ ^ / +',
    '1 2 + 3 4 + ^ 5 6 + ^';

Output:

3 4 2 * 1 5 - 2 3 ^ ^ / +
3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3

1 2 + 3 4 + ^ 5 6 + ^
( ( 1 + 2 ) ^ ( 3 + 4 ) ) ^ ( 5 + 6 )