In general, you can't when using a foreach() loop. In specific instances, you can use join() (when joining a string), shift elements off the array and check its length, or or use an iterator object to abstract it all away:

while (my $value = $it->next()) {
  # stuff
  if ($it->has_next()) {
     # for all but the last value
  }
}
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update: equivalent with shift

while (@array) {
  my $value  = shift @array;
  # stuff
  if (@array) {
     # for all but the last value
  }
}
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update2: fixed logic error: s/unless/if/; - sorry about that.