Warnings via Perl’s -w Switch
Let’s go through the script section by section.
First comes the usual top-of-the-script stuff, with one
change—the shebang line now has a trailing
-w:
#!/usr/local/bin/perl -w
As mentioned in Chapter 2 in the discussion of
debugging,
-w turns on Perl’s
warnings feature, which causes the script to
complain to standard error if certain suspicious-looking things
appear to be going on. We haven’t bothered with it before this,
since the other scripts so far have been so short and simple, but
this one is complex enough that it’s worth turning it on.
With that said, you should realize that Perl’s warnings feature is mainly a tool to help you while you’re writing the script. Once the script is written and working properly, there shouldn’t be any warnings.
Tip
Beginning with Perl Version 5.6.0, you can enable warnings by putting
a statement that says use warnings; near the
beginning of your script instead of using the -w
shebang-line switch. If your version of Perl is recent enough to
support it, the use warnings approach has some
minor advantages over the -w switch, so you should
probably use it. In this book I’ll just be using the
-w switch.