In the "Other I/O" section of the MIME::Body documentation, the following examples are shown: 

 ### Dump the ENCODED body data to a filehandle:
 $body->print(\*STDOUT);

 ### Slurp all the UNENCODED data in, and put it in a scalar:
 $string = $body->as_string;

 ### Slurp all the UNENCODED data in, and put it in an array of lines:
 @lines = $body->as_lines;

[download]

Why not, then, do something such as this (untested)? 

    for $part( @parts ) {
        my $type = $part->mime_type;
        my $bh   = $part->bodyhandle;
        print "MIME Type: $type\n";
        if ( defined $bh ) {
            open( my $OUTFILE, ">", "output.7z" ) or die $!;
            $bh->print(\$OUTFILE);
            close( $OUTFILE );
        }
    }

[download]

Also, have you looked at the output.7z file that was generated to see if perchance it was encoded (Base64, Quoted-Printable, etc.), or if there was perhaps miscellaneous characters at the beginning causing 7zip to think the file invalid?

Hope that helps.

Update: 2015-07-09
Added additional question regarding output.7z file.

In reply to Re: MIME::Tools to save attachment properly by atcroft
in thread MIME::Tools to save attachment properly by gcasa

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