'can\'t figure out file type, specify explicitly'

You need to specify the format. (Seems to be a Windows thing only.) So, I would do:

my $piddle = PDL->rpic($path, {FORMAT => 'PNG'});
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which works fine for me (iff pngtopnm.exe/pnmtopng.exe are in my path). I'm not familiar with the rim() function - afaict

my $piddle = rim($path, {FORMAT => 'PNG'});
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should also work, but for me that just produces:

Can't locate object method "dim" via package "file.png" (perhaps you f
+orgot to load "file.png"?) at C:/MinGW/perl516/site/lib/PDL/IO/Pic.pm
+ line 570.
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Update: the rim() function is broken in my build of PDL (version 2.4.11), and may well have been broken for quite some time.
It just calls rpic() anyway, so use that instead.

(For my test script, $path was set to file.png ... obviously.)

Cheers,