[% BLOCK first_block -%]
this is my first block, a is set to '[% a %]'
[%- END -%]
[% BLOCK second_block; DEFAULT b = 99 m = 98 -%]
this is my second block, a is initially set to '[% a %]' and 
then set to [% a = s %]'[% a %]'  b is $b  m is $m
[%- END -%]