Documented behaviour:
(...) if FILEHANDLE is an expression, its value is the real filehandle. (This is considered a symbolic reference, so use strict "refs" should not be in effect.) 

system 'echo abc > 1.txt';
open '<:raw', '1.txt' or die $!;
print while readline '<:raw';
unlink '1.txt';

[download]
Update: I wouldn't recomend it, though, as it's confusing to a reader :-) The file is still opened with default layers: 
print PerlIO::get_layers("<:raw");
# unixperlio

[download]
لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re^2: Using pack/unpack on a PNG file by choroba
in thread Using pack/unpack on a PNG file by sierpinski

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