#!/usr/bin/perl

$n=25;
$m=15;

$A[$n-1] = undef;         # autovivify @A to length $n

sub foo  {
    my ($aref, @i) = @_;
    @$aref[@i] = (42) x @i;
    return;
}

foo(\@A, grep { 0 == $_ % 2 } 0 .. $m);

use Data::Dumper;
print Dumper(@A);