you didn't tell us that you need only an interval out of the array, the fastest approach with my version of Perl is iterating over an interval or array slice

  DB<173> use Time::HiRes qw/time/

  DB<174> @x=();$t=time; for (my $i=$start; $i<=$end;$i++) { push @x, 
+$a[$i]}; print time-$t
0.21531081199646

  DB<175> @y=(); $t=time; push @y, $_ for @a[$start..$end]; print time
+-$t
0.1353440284729

  DB<176> @z=();$t=time; push @z, $a[$_] for $start..$end; print time-
+$t 
0.142512083053589
[download]

(you are welcome to do proper benchmarking =)

but optimizing or inlining &$keyGen( $element, @_ ) should lead to much more efficiency, since 25% of 15% doesn't count much!!!

update

of course you could directly iterate over the values of a hash slice of an array slice... :)

  DB<180> %h=(); $i=$start; $t=time; push @$_, $i++ for @h{@a[$start..
+$end]}; print time-$t
 => 1
0.427901983261108
[download]

BTW: @a=0..1e6;$start=1e5;$end=2*$start;