note friedo <p>They're similar, but not exactly the same. Putting <c>-w</c> in the command line (or the shebang) will turn on warnings globally, for all the Perl code loaded by that process.</p> <p>On the other hand, <c>use warnings</c> is a "lexical pragma," and only applies in the block in which it's used. If you put it at the top of a script or module, then it will apply for that entire file (this is called file scope.) Inside a block, it applies only for that block: <c> #!/usr/bin/perl print "no warnings out here!" . $undef_var; { use warnings; # now warnings are turned on print "this will warn" . $undef_var; } # now warnings are off again. </c> <p>(Note: Try running that with and without <c>-w</c> to see the difference.</p> <p> Typically, it's a good idea to <c>use warnings</c> in your file scope, which allows you to occasionally turn them off in small blocks (using <c>no warnings</c>) if you absolutely must do something that will generate a warning. See [doc://perllexwarn] for the gory details. </p> <i>And what are things like the '-w' called (eg the -f or -d to specify file or directory)?</i></p> <p> Those are called command-line switches or arguments, and perl's are all documented in [doc://perlrun]. </p> 695165 695165