$text =~ s/
        [ ]+ |                  # spaces, or:
        (                       # begin $1
            STARTPRESERVE(\d?)  #     STARTPRESERVE + $2
            .*?                 #     anything
            STOPPRESERVE\2      #     STOPPRESERVE + $2
        )                       # end $1
    /${ [$1,' '] }[ ! $1 ]/sgx;

# [$1,' '] is an unnamed array.
# ! $1 is:
#     0 if $1 exists;
#     1 if spaces were found.