my $str = 'showParams: "{"revertType":null,"refresh":false,"checkpoint":false,"sheetName":"","unknownParams":"zone_id=6290&ac=a9aef098062f9646bd1638d0c6054c7b&account_id=62&Start_Date=2016-07-27&End_Date=2016-08-03","layoutId":""}"';
while ($str =~ /(?<!\\)"\s*\{\s*("(?:(?!\}").)*)\}"/) {
    substr($str, $-[1], $+[1] - $-[1]) =~ s/(?=["\\])/\\/g;
}