If you want to use hashrefs, and choose to ignore tye's advice (in favor of speed?), you can get also the key values using hv_fetch:

int
calculate(hashref)
  SV * hashref;
INIT:
  HV * myhash;
  SV **svp;
  int keylen;
CODE:
char * name;
float theta;

  myhash = (HV *)SvRV(hashref);
  svp = hv_fetch(myhash,"NAME",4,0);
  if (svp) name = SvPV(*svp,keylen);
  svp = hv_fetch(myhash,"THETA",5,0);
  if (svp) theta = SvNV(*svp);
  printf("%s,%.1f\n",name,theta);
    .
    .
    .
  RETVAL = 1;
OUTPUT:
  RETVAL
[download]

If you're determined to use hashes, check out perlguts, and the book "Advanced Perl Programming" by Sriram Srinivasan.

- robsv