Use strict and warning

#!/usr/bin/perl

Today I read a perl script, I saw the "use strict" and "use warnings" in the head lines of content.

I learned about it's function and using method.  I would write it here.

Put this line at the top of your script (after the shebang, e.g., '#!/usr/bin/perl' line):

use strict;

Whenever you first use a variable, put 'my' in front of it.

In version 5.6 or later you can put this right around the same place you put 'use strict;':

use warnings;

In perl's before 5.6 (or if you just want to be portable between the versions), you can put '-w' on the 'shebang' line, or set the $^W variable (however, setting $^W will not catch compile time warnings unless its in a BEGIN{} block, so '-w' is usually preferable):

#!/usr/local/bin/perl -w

# Or
$^W = 1;
# Or
BEGIN { $^W = 1 }

If you want to disable the warning in some limited scope.

1

sub add_two_numbers_which_might_be_undef 

{
 # See 'perldoc perllexwarn' for all the categories of warnings
 # because its better to only disable the warnings you're expecting
 no warnings "uninitialized";
 $_[0] + $_[1];
}

2
sub add_two_numbers_which_might_be_undef 

{
 local $^W;
 $_[0] + $_[1];
}