my $element_size = 3;
my $group_size = 10; #adjust as needed for 5th, 7th, etc.
my $block_size = $element_size * $group_size; # since "elements" are virtual

my @new;

while ( @old_array ) {
    if ( @old_array < $block_size ) { # don't add new if old is too small
        push(@new, @old_array);
        last;
    }
    push(@new, splice(@old_array,0,$block_size); # "shift" out of old
    push(@new,qw( create mount remove )); # new "element" added here
}

# UPDATE: added code to put remainder of @old_array onto @new if not
# a full multiple.