In addition to the excellent comments by castaway above, I would only add that it is additionally advantageous to include an escape clause within the recursive subroutine in order to ensure that the parsing routine does pass beyond a specified recursion depth - This is a relatively simple safeguard to implement from a defensive programming standpoint.

For example, a one-liner which (eventually) exhausts the memory available to the Perl process:

rob@development:/home/rob/workspace$ perl -e 'sub a { &a }; a'
Out of memory!
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This can be prevented by the incorporation of a maximum depth for subroutine recursion such as follows:

sub parsevcard {

    my ($inputstr, $inputpos, $depth) = @_;
    if ($depth > $max_depth) {

        #   Recursion beyond allowable depth error handling here
    }

    .
    .

    $currentcard->{AGENT} = parsevcard($inputstr, $pos, ++$depth);
}
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Depending upon the recursive problem at hand, this error trapping can be performed by way of examination of the output data structure alone, for example, the number of elements within an output array, rather than by a separate counter per se.