$sublable->insert( 'end', $_ ) for @contents ;
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$sublable->insert( 'end', join('',@contents) );
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works fine for me, too, because - unlike your array interpolation in double quoted strings with default setting of $" - this does not add spaces.

As for

$sublable->insert( 'end', @contents );
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behaving strangely: the synopsis for the insert method is

$text->insert(index, chars, ?tagList, chars, tagList, ...?)
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so when you pass a list of arguments, every other of its elements is being interpreted as a "tagList" parameter. I think this explains what you're getting... No encoding mysteries involved.