My guess is that it is related to Perl not completely throwing away a lexical variable if it goes out of scope, because you often reenter a block and it's faster to keep the structure around. Look at this:

my @a =  1 .. 10;
my @b =  5 .. 15;
my @c = 10 .. 20;
my @d = 15 .. 25;

for (1 .. 2) {
    @B = do {local %h;
                   $h {$a [0]} ++;
                   print "B: ", scalar %h, "\n";
                 ++@h {@a, @b, @c, @d};
              keys %h};
    @C = do {   my %h;
                   $h {$a [0]} ++;
                   print "C: ", scalar %h, "\n";
                 ++@h {@a, @b, @c, @d};
              keys %h};
}
__END__
B: 1/8
C: 1/8
B: 1/8
C: 1/32
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The second time it's executing the my %h block, the hash already has 32 buckets.

Abigail