With the example you have given, i say point A, because if you declare your array at point B, you will re-declare it as many times as you call mySub(). Also, consider passing @bigarray to mySub() as a reference. Just be sure to declare mySub() before @bigarray, otherwise @bigarray is accessible by mySub():

use strict;

sub mySub {
   my $ref = shift;
   for (@$ref) {
     #do stuff with array
   }
}

my @bigarray = ("insert", "a very", "long list here");

my $thing1 = mySub(\@bigarray);
my $thing2 = mySub(\@bigarray);
[download]

If the array in question is only pertinent to the subroutine, and either that sub will only be called once or the array will change with each sub call, then declare the array inside the subroutine.

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)