You need to prepend the directory to the filename (see readdir), and you can check if the read was successful: 
my $dir = $vars{to_images};
...
my $err = $image->ReadImage("$dir/$_");
warn "$err" if $err;

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Also, might be better to filter out the non-image files before passing to Image::Magick.

In reply to Re^3: Using ImageMagick effectively by tangent
in thread Using ImageMagick effectively by Aldebaran

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