To repeat the behaviour of your code I'd do: 
my @mails;
while (<>) {
  if (/^From /) {
    $mails[$#mails + 1] = $_;
  }
  else {
    $mails[$#mails] .= $_;
  }
}

[download]
It seems to me from previous experimentation that any lines in the body text starting with 'From ' will already be quoted with a > character, so that they will not cause the test I used to pass and will not therefore create a new array element.

In reply to Re: stream parsing an mbox file by repson
in thread stream parsing an mbox file by johnfoobar

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