Your code: 

my @fifth =  @arr[ map {$_*5-1} 1..$#arr+4/5 ];

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adds a fraction (4/5) to the last array index. So adding appropriate parentheses: 
my @fifth =  @arr[ map {$_*5-1} 1..($#arr+4)/5 ];

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seems better but it does not work for all array lengths. (It works if the length modulo 5 is 0 or 1 only.) Changing the 4 to a 1 (aka the array length) does the trick: 
my @fifth =  @arr[ map {$_*5-1} 1..($#arr+1)/5 ];

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In reply to Re^2: how to push 5th element from one array to another array by gmargo
in thread how to push 5th element from one array to another array by Anonymous Monk

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