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expr      := number
           | expr ws unary_operator 
           | expr ws expr ws binary_operator

RULE1 = RULE2 | RULE3
RULE2 = A B
...
RULE1x = A ( RULE2x | RULE3x )
RULE2x = B
RULE3x = C

Parse "1 2 +" as an expr
 Try 'number'
...
    Parse 'end', PANIC!
   Try 'expr unary'
   ...

Parse "1 2 +" as an expr
 Try 'number'
...
  Parse 'number', left with "+"
  Parse 'binary', left with ""
  Parse 'end', done!

my %ops = (
    '+' => sub { $_[0] + $_[1] },
...
#sx;

(reverse $line) =~ $RPN3;