high water mark 算法（返回最大值）

use warnings;
sub max{
 my $num;
 foreach (@_){
  if ($num < $_){
   #print $num, "\n";
   $num = $_;
  }
 }
 $num;
}
@numbers = qw /342 6436 43 1 -13 55555 431/;
print "The max number is ", &max(@numbers), ".\n";

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Print log is as below:

C:\Perl\learn>"high-water mark.pl"
Use of uninitialized value $num in numeric lt (<) at C:\Perl\learn\high-water ma
rk.pl line 5.
The max number is 55555.