As far as I can see it's not used by any template operators, so your code there probably generates a debug warning that there is no template operator named pow.
But you can use a setting called PHPOperatorList in template.ini* to register the php pow** function:
[PHP]
PHPOperatorList[phppow]=pow
And use it like:
{def $nine = 3|phppow(3)}
{$nine}
* place the setting in settings/override/template.ini.append.php ** http://no2.php.net/pow
You have probably reached the ceiling of integer type*. But that can probably be avoided if you fix your code to do what I showed above (you used the example from before I updated it).
* http://no.php.net/int
The size of an integer is platform-dependent, although a maximum value of about two billion is the usual value (that's 32 bits signed).