# init variable to show we're not in the blcok
my $in_block = 0;

while ( <> ) # process line by line
{
    $in_block = 1 if /^head/; # test for start of block and
                              # set flag true if needed
    print if $in_block; # print if we're in the block
    $in_block = 0 if /tail$/; # test for end of block and
                              # set flag false if needed
}