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my @filtered = grep { condition($_) } @elements;

my @indexes = grep { condition($elements[$_]) } 0..$#elements;

my @filtered;
for (@elements) {  # Not any list, an array specifically.
   push @filtered, $_ if condition($_);
}

my @indexes;
for (0..$#elements) {
   push @indexes, $_ if condition($elements[$_]);
}