$_ = '0   1   
STARTPRESERVE
    STARTPRESERVE
        2 
    STOPPRESERVE
STOPPRESERVE
3   4
STARTPRESERVE
    STARTPRESERVE
        5
    STOPPRESERVE
STOPPRESERVE
6   7';

(my $re=$_)=~s/((STARTPRESERVE)|(STOPPRESERVE)|.)/${['(','']}[!$2]\Q$1\E${[')','']}[!$3]/gs;
$re= join'|',map{quotemeta}eval{/$re/};
die $@ if $@ =~ /unmatched/;
s/\s+|($re)/${[$1,' ']}[!$1]/g;