use XML::Parser;
...
my $p = XML::Parser->new(Style => 'Subs',...);

my %AccountLocations;
my $CurrentAccount;

$p->parsefile($xmlfile);

# Handler for <Account> start element
sub Account {
  my ($parser, $tag, %attributes) =@_;


  # store the byte-offset into the file of the required node
  if (SomeConditionMet(%attributes) {
    # remember it so we can use it for the closing tag
    $CurrentAccount = $attributes{Number}; 
    $AccountLocations{$CurrentAccount}->{Start} => $p->currentbyte()};
  }
}

# Handler for </Account> end element
sub Account_ {
  my ($p, $tag) = @_;

  # XML::Parser::current_byte is the offset in the file to the start
  # of the node. Add the length of what the parser
  # recognised to get the offset to the end of the node

  # is this the closing tag for a required account ?
  if (defined $CurrentAccount) {
    $Accounts{$CurrentAccount}->{End} = 
      $p->current_byte() + length($p->recognized_string());
    $CurrentAccount = undef;
  }
}

sub SomeConditionMet {
  my (%attributes) = @_;

  # your code goes here
  ...
}