You can randomly use pop and shift to remove elements one at a time from either end of your array and add them to the sum, doing this in a loop while there are still elements in the array. However, this obviously destroys your array.

$ perl -Mstrict -Mwarnings -E '
> my @arr = ( 1 .. 10 );
> my $sum;
> $sum += int rand 2 ? shift @arr : pop @arr while @arr;
> say $sum;
> say qq{-> @arr <-};'
55
->  <-
$
[download]

If, on the other hand, you wish to preserve your array, do the same thing in an on-the-fly subroutine with your array as the argument, which is passed into the subroutine as the @_ array (see perlvar and perlsub). The pop and shift functions inside a subroutine operate on @_ by default, leaving your @arr array untouched.

$ perl -Mstrict -Mwarnings -E '
> my @arr = ( 1 .. 10 );
> my $sum = sub {
>    my $sum;
>    $sum += int rand 2 ? shift : pop while @_;
>    return $sum;
>    }->( @arr );
> say $sum;
> say qq{-> @arr <-};'
55
-> 1 2 3 4 5 6 7 8 9 10 <-
$
[download]

I hope this is helpful.

Update: Clarified the wording a little.

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