Ok, here it goes with these specifications:

%Marks = (
  STARTPRESERVE => 'STOPPRESERVE',
  BEGIN => 'END' );
$MarkOptions = join("|",keys %Marks);

$text =~ s/
        \s+ |                   # spaces, or:
        (                       # begin $1
            ($MarkOptions)(\d?) #     $2=START + $3=optional_number
            .*?                 #     anything
            $Mark{\2}\3         #     $Mark{START} + optional_number
        )                       # end $1
    /${ [$1,' '] }[ ! $1 ]/sgx;

# [$1,' '] is an unnamed array.
# ! $1 is:
#     0 if $1 exists;
#     1 if spaces were found.
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Note: you may add \b to the markers if you feel that you need it.
I kept the optional number after the marker, you may remove it if you don't need it.