I don't know much about HTML::Template, but what you are passing it is a one-element array that contains a hash reference. I suspect you either want an array of the keys and values (my @loop_data = %hash), or an array of hash refs, in which case you probably just want to build it directly instead of building a hash first.

# Instead of
# $hash{$img_number} = $img_user;
#
# Try
push(@loop_data, {$img_number => $img_user});
# or
push(@loop_data, {
    img_number => $img_number,
    img_user => $img_user,
});
# depending on what your template is expecting to see
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This way you end up with an array of hashes, each hash containing the data for one image.