Just thought I'd try my hand at creating the drop dead most simple examples I could. Doesn't handle errors as nicely. 
#Perl5
$rpn = "3 10 * 20 5 / +";

$dig = qr/-?\d+\s+/;
$op  = qr/[+\-*\/]/;
%f = ("+" => sub{$_[0] + $_[1]},
      "-" => sub{$_[0] - $_[1]},
      "*" => sub{$_[0] * $_[1]},
      "/" => sub{$_[0] / $_[1]});

1 while($rpn =~ s/^(.*?)($dig)($dig)($op)/$1.($f{$4}->($2,$3))/e);

print "$rpn\n";

[download]
...and... 
//Haskell
main = print $ foldl eval [] $ words "3 10 * 20 5 / +" -- 34

eval (x:y:xs) "+" = y+x : xs
eval (x:y:xs) "-" = y-x : xs
eval (x:y:xs) "*" = y*x : xs
eval (x:y:xs) "/" = y`div`x : xs
eval  xs      dig = (read dig):xs

[download]

In reply to Re: Tales from writing a RPN evaluator in Perl 5, Perl 6 and Haskell by Anonymous Monk
in thread Tales from writing a RPN evaluator in Perl 5, Perl 6 and Haskell by eyepopslikeamosquito

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