1. if you post code that uses some complex data structures, provide an example, e.g.

 
my @UnNum = ( [ 0, [1,2,3] ], [ 0, [1,2,3] ]);
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2. if you don't understand some code others post here, try to read the perldoc (e.g. perldoc -f map) and then ask again if sth is still unclear.

what this join/map line does (from inner to out):

join ' ', @{$_->[1]}
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Simply generate a string with all elements in the second element (u know what i mean ;) ), the arrayref, seperated by blank.

map {"$_->[0]\t " . join ' ', @{$_->[1]}} @UnNum;
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now generate an array with the same number of elements like @UnNum. the elements are now a string where the first array element is seperated by a tab from the string generated with join.

print join "\n", map {"$_->[0]\t " . join ' ', @{$_->[1]}} @UnNum;
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now generate a string out of this new array, seperated by newline and print it.

if you understand this, then you know the answer to your question.