my $mail = Email::Simple->new($msg);
my (%headers_i_care_about);
foreach my $i_care_about 
    (qw(Received Date From To Subject Return-Path)) {

  my @{$headers_i_care_about{$i_care_about}} = 
      $mail->header($i_care_about);

  # or, if you want it as a single string, something like:
  # my $headers_i_care_about{$i_care_about} = 
  #     join(' ', $mail->header($i_care_about));
}

##</code><code>##

my (%headers);
my ($lastheader);
foreach my $line (@msg) {
  last if ($line =~ m/^\s*$/);  # Reached end of headers
  if ($line =~ m/^\s+(.+)/) {
    $headers{$lastheader} .= $1;
  } else {
    my @parts = split(/:/, $line, 2);
    $headers{$parts[0]} = $parts[1];
    $lastheader = $parts[0];
  }
}