How to I convert a string to a number

#How to I convert a string to a number

sub atoi {
 my $t;
 foreach my $d (split(//, shift())) {
     $t = $t * 10 + $d;
     print "$t\n";
  }
return $t;
}

$number = atoi("123");

print "string '123' number value is $number";