Difference between the use warning command and-W in Perl

The following two commands can be used to check insecure syntax in Perl programming:

 
#! /Usr/bin/perl-W
 

Or:

 
Use warnings;
 

Although both of them can prompt insecure syntax, there is still a difference. The Perl help documentation writes:

The warnings Pragma is a replacement for the command line flag-W, But The Pragma is limited to the enclosing block, while the flag is global.

It can be seen that the scope of the former is global, while the latter can be terminated by the following statement in advance:

 
No warnings;