qwurx [shmem] ~> perl 574821.pl 6000 7000 10000
Array size: 10000
Inserting: 6000 .. 7000
           Rate splicing  radiant
splicing 46.8/s       --     -66%
radiant   137/s     193%       --
[download]

You are inserting a number between 6000 and 7000 at index 1, in every call to insert1 and insert2.

# called as insert1( \@ary, 1, $_ ) for $START .. $END
sub insert1 {
  my ( $ra, $index, $elem ) = @_;
  @$ra = @$ra[0 .. $index-1], $elem, @$ra[$index .. @$ra-1];
}
# I tested with 
sub insert1 {
  my ( $ra, $elem, $index ) = @_;
  @$ra = @$ra[0 .. $index-1], $elem, @$ra[$index .. @$ra-1];
}
# called as insert1( \@ary, 1, $_ ) for $START .. $END
[download]

If I swap $elem and $index (i.e. insert 1 at an index from $START to $END) I get:

qwurx [shmem] ~> perl 574821.pl 600 700 1000 
Array size: 1000
Inserting: 600 .. 700
           Rate  radiant splicing
radiant  30.5/s       --     -98%
splicing 1672/s    5377%       --
[download]

The insert somewhere in the middle is more expensive, that's why $_/10 for @params here.

My perl:

$ perl -v
This is perl, v5.8.8 built for i586-linux-thread-multi
...
[download]

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}