Since $a is defined in the same package - main - what happens is that the value of $a gets merely overwritten, variables in the same package are global to that package scope,

#!/usr/local/bin/perl
print  __PACKAGE__ ;
print "\n";
$a=10;
{
  $a=11;
  print $a,"\n";
  print  __PACKAGE__ ;
}
[download]

look up __PACKAGE__ in perlmod and perldata, coping with scoping can be a good read as well..