The (explicitly stated or implicite $_) loop variable in foreach loops is always an alias (what Perl calls C-like references) to the element, as opposed to a copy. This has nothing to do with a reference ($a) being involved. If you want to modify the loop variable without modifying the array element, use one of the following workarounds:

foreach (@array) {
   my $i = $_;  # $i is a copy.
   ...
}
[download]

or

foreach (@array) {
   local $_ = $_;  # $_ is a copy.
   ...
}
[download]