You can do this much more efficiently, O(1), by using a lookup table.

my @lookup = ( '00', 
    map {
        ( $_->[2] ) x ( $_->[ 1 ] - $_->[ 0 ] )
    }   
    [  0,   3, '00' ], [  3,   7, '05' ], [  7,  15, '10' ],
    [ 15,  25, '20' ], [ 25,  35, '30' ], [ 35,  45, '40' ],
    [ 45,  55, '50' ], [ 55,  65, '60' ], [ 65,  75, '70' ],
    [ 75,  85, '80' ], [ 85,  93, '90' ], [ 93,  97, '95' ],
    [ 97, 100, '99' ]
);

print "@lookup";

$rv .= $lookup[ ((( $prob - 0.5 )*2 )+( $conf * 100 )) ];
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