$_= '1 <% xxx%%> 2 <%%> 3 <%>%> 4 <% >% %> 5 <%%%% xxx %%%%> 6 '; 

s[ <% .*? (?> %> ) ][!REPLACED!]xg; 

print;

1 !REPLACED! 2 !REPLACED! 3 !REPLACED! 4 !REPLACED! 5 !REPLACED! 6