I linked to the documentation of the modulo operator. Your subroutine basically seems to do:

sub mod_9 {
    my( $x ) = @_;
    return $x % 9
}
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Update: No, I misread the original function. You seem to input a number with decimals and want to know into which 9-tile it falls. int ($x*9)-1, as LanX notes below, is the correct answer then.