Seconded on the benchmark first suggestion. Baring that it'd be trivial to memoize the converted values if there's a high likelyhood that they'll show up repeatedly (then again you're wagering that the hash lookup and deref will be faster than 3 shifts and bitwise-ands, so again benchmark).

my %_pixel_as_rgb;
for my $pixel ( @allpixels ) {
  push @rgbvals,
    @{ $_pixel_as_rgb{ $pixel } ||= [ $pixel >> 16 & 255, $pixel >> 8 
+& 255, $pixel & 255 ] };
}
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