The section from perllexwarn that you might find most interesting comes under the headline, "What's wrong with -w and $^W":

Although very useful, the big problem with using -w on the command line to enable warnings is that it is all or nothing. Take the typical scenario when you are writing a Perl program. Parts of the code you will write yourself, but it's very likely that you will make use of pre-written Perl modules. If you use the -w flag in this case, you end up enabling warnings in pieces of code that you haven't written.

The section then goes on to describe how $^W falls short of providing lexical scoping for -w.

For simple code snippets, -w and use warnings are synonymous. But for more elaborate code, and scripts that use modules, or scripts that need lexical control over warnings, the pragma is preferable over the command-line switch.

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