# Start block 0
use strict;

my $foo;          # exists from here to the end of the file

{ # Start block 1
     my $a;       # exists from here to end of block 1

     { # Start block 2

           my $b; # exists from here to end of block 2

           print $b;

     } # End block 2: $b stops existing here 

     # print $b; # (would fail, no $b can be found)     
     $a++;

} # End block 1: $a stops existing here

# print $a;  # (would fail, no $a can be found)
# print $b;  # (would fail)
print $foo;  # successful

# End of file $foo stops existing here.