my $string = q(
 Outside.
 {tag}
 Inside level 1.
 {tag}
 Inside level 2.
 {/tag}
 Inside level 1.
 {/tag}
 Outside.
 );
(my $re=$string)=~s/((\{tag\})|(\{\/tag\})|.)/${[')','']}[!$3]\Q$1\E${['(','']}[!$2]/gs;
@$ = (eval{$string=~/$re/});
die $@ if $@=~/unmatched/;
$re =  join("|",map{quotemeta}@$);
print $string while $string=~s/\{tag\}($re)\{\/tag\}/--Marked--\n$1\n--EndMarked--/;