The thing is that within "perl -d", each line you type interactively is effectively in it's own block. Try the following:

  DB<1> my $x = 1

  DB<2> p $x

  DB<3> $x = 1

  DB<4> p $x
1
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Notice how the first initialization of $x did not persist, since that $x was a lexical, and hence confined to the scope of that line. The second initialization of $x did persist, however, because this $x is in fact $main::x:

  DB<5> p $main::x
1
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Update: One other consequence of the fact that every line you type lives in its own block is that global variables that are implicitly localized to the enclosing scope (i.e. regexp-matching variables like $&, $', $`, $1, $2, $3...) will not survive from one line you type to the next. For example

  DB<1> 'a' =~ /(.)/

  DB<2> print $1

  DB<3> 'a' =~ /(.)/ && print $1
a
  DB<4>
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Therefore, if you want to be able to use one of these variables over several lines, assign it to a package global:

  DB<5> 'a' =~ /(.)/ && $g = $1
  DB<6> print $g
a
  DB<7>
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Update: Added some emphasis.