You need to upgrade libwww-perl to the latest version (5.800). From the changelog: 
2004-04-07   Gisle Aas <gisle@ActiveState.com>

    Release 5.78
    ...   
    Added clear() and remove_content_headers() methods to
    HTTP::Headers.

[download]

In reply to Re: HTTP::Headers error when submitting form via WWW::Mechanize by drewbie
in thread HTTP::Headers error when submitting form via WWW::Mechanize by jerrygarciuh

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