UPDATE

As per my revelations in sort this data, here is a bit of an adjusted report on the av.c source.

Here's the relevant portion from av.c in the 5.6 source:

/* this is Perl_av_unshift()
   it unshifts 'num' undef values to an array */

/* determine how much non-used spaced is left
   that's been allocated for this array */
i = AvARRAY(av) - AvALLOC(av);

/* if there's room left... */
if (i) {
  /* if there's more room than we need, just use 'num' */
  if (i > num) i = num;

  /* this will set 'num' to 0 if we had enough room */
  /* 'num' is now how many new undef values we need added */
  num -= i;
  
  AvMAX(av) += i;  /* set the highest subscript??? */
  AvFILLp(av) += i;  /* add to highest subscript */
  SvPVX(av) = (char*)(AvARRAY(av) - i);  /* where Perl's array starts 
+*/
}

/* if there wasn't enough room already... */
if (num) {
  i = AvFILLp(av);  /* highest subscript */
  av_extend(av, i + num);  /* extend array to i+num elements */
  AvFILLp(av) += num;  /* add to highest subscript */
  ary = AvARRAY(av); /* get at the array */
  Move(ary, ary + num, i + 1, SV*);  /* slide elements up */
  do {
    ary[--num] = &PL_sv_undef;  /* set new elements to undef */
  } while (num);
}
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Ok, so unshift() has to do a lot of sliding the elements up if there's not already some free space generated by a call to shift().

What shift() does is simply slide the pointer up one notch, which is very fast (which provides free space for unshift() later). It stores the value in *AvARRAY(av) into retval, and then sets *AvARRAY(av) to the C equivalent of Perl's undef value. Then it moves the pointer that marks the beginning of the array to AvARRAY(av) + 1. It also subtracts one from the max subscript and the length of the array. Then it returns retval.

<revelation>I'm beginning to grok the source code! Look out!</revelation>.

japhy -- Perl and Regex Hacker