From the Imager::Files pod, it appears that, $img->read( data => $temp, type => 'pnm'); should work. You might consider just moving through the in-memory image with something like,

my $finger = 0;  # or wherever the data starts
while ($finger < length $temp) {
    substr( $temp, $finger, 3) =
        pack 'C3',
            reverse unpack 'C3', substr $temp, $finger, 3;
    $finger += 3;
}
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Untested.

Update: corrected code to assume 24-bit color.

After Compline,
Zaxo