If you actually want "the nearest multiple of 5", then:

sub roundToNearest5{  
   5 * int( ( $_[0] + ( 2 * ( $_[0] <=> 0 ) ) ) / 5 );
}

print "$_: ", roundToNearest5( $_ ) for -18..+18;;
-18:  -20
-17:  -15
-16:  -15
-15:  -15
-14:  -15
-13:  -15
-12:  -10
-11:  -10
-10:  -10
-9:  -10
-8:  -10
-7:  -5
-6:  -5
-5:  -5
-4:  -5
-3:  -5
-2:  0
-1:  0
0:  0
1:  0
2:  0
3:  5
4:  5
5:  5
6:  5
7:  5
8:  10
9:  10
10:  10
11:  10
12:  10
13:  15
14:  15
15:  15
16:  15
17:  15
18:  20
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