#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";