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

sub arith($);

my %h = (
        'add'   => '+',
        'sub'   => '-',
        'mul'   => '*',
        'div'   => '/',
);

print arith(eval $ARGV[0]), "\n";

sub arith($) {
        my $a = shift;
        ref $a ? eval join $h{$a->[0]}, map arith($_), @{$a}[1..$#$a] : $a;
}

__END__

call it with listrefs:

['add', 3, ['mul', 8, 9], ['sub', ['div', 16, 4], 5]]