It's not Perl, but you might consider using formail for this. A snippet from the formail manpage:

    NAME
       formail - mail (re)formatter

    [...]

    DESCRIPTION
       formail is a filter that can be used to  force  mail  into
       mailbox  format,  perform `From ' escaping, generate auto-
       replying headers, do simple header  munging/extracting  or
       split  up  a mailbox/digest/articles file.
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