#Okay, you asked for it:

$_ = '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;
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