Actually the size of the array shouldn't matter because when foreach gets a real array perl will be smart and won't expand things in place (it uses an iterator over the AV* behind the scenes which all happens in C underneath).

$ perl -MBenchmark=timethese -e <<'EOC'
$a = [ 0 ..5_000 ];
timethese( 2_000,
  { index => sub { for( my $i = 0; $i <= $#{ $a }; $i++ ) { $b = $a->[
+$i] } },
    iterator => sub { for( @{ $a } ) { $b = $_ } } } )
EOC
Benchmark: timing 2000 iterations of index, iterator...
     index: 27 wallclock secs (21.21 usr +  0.02 sys = 21.23 CPU) @ 94
+.21/s (n=2000)
  iterator:  6 wallclock secs ( 5.39 usr +  0.00 sys =  5.39 CPU) @ 37
+1.06/s (n=2000)
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