OK ... now for a quick round of golf. Here's a version that's a bit tighter and easier to expand:

#!/usr/bin/perl -w
use strict;
use warnings;

my @stack = ();

sub binop {
        my ($op,   $prec)   = @_;
        my ($rhop, $rhprec) = @{pop @stack};
        my ($lhop, $lhprec) = @{pop @stack};

        # Wrap operand(s) with lower precedence
        $lhop = '(' . $lhop . ')' if $prec > $lhprec;
        $rhop = '(' . $rhop . ')' if $prec > $rhprec;

        push @stack, [ $lhop . $op . $rhop, $prec ];
}

my %handlers = (
        'push'  => sub { push @stack, [shift, 999]; },
        'print' => sub { print "out: '" . ${@{pop @stack}}[0] . "'\n";
+ },
        'add'   => sub { binop '+', 1; },
        'mul'   => sub { binop '*', 2; }
);

while (<DATA>) {
        chomp;

        print "TOS='" . (@stack ? $stack[$#stack][0] : '-nil-') . "' "
        .       "stmt = '" . $_ . "'\n";

        my ($operator, $operand) = split;
        if ($handlers{$operator}) {
                $handlers{$operator}->($operand);
        }
        else {
                print "Unknown operator: '$operator' for line '$_'\n";
        }
}
__DATA__
push 3
push 5
add
push apples
add
push oranges
push 9
push bananas
push peels
mul
add
add
mul
print
[download]

Which gives us:

TOS='-nil-' stmt = 'push 3'
TOS='3' stmt = 'push 5'
TOS='5' stmt = 'add'
TOS='3+5' stmt = 'push apples'
TOS='apples' stmt = 'add'
TOS='3+5+apples' stmt = 'push oranges'
TOS='oranges' stmt = 'push 9'
TOS='9' stmt = 'push bananas'
TOS='bananas' stmt = 'push peels'
TOS='peels' stmt = 'mul'
TOS='bananas*peels' stmt = 'add'
TOS='9+bananas*peels' stmt = 'add'
TOS='oranges+9+bananas*peels' stmt = 'mul'
TOS='(3+5+apples)*(oranges+9+bananas*peels)' stmt = 'print'
out: '(3+5+apples)*(oranges+9+bananas*peels)'
[download]

--roboticus