#!/usr/bin/perl -w and use warnings;

ghenry has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: #!/usr/bin/perl -w and use warnings; by dave_the_m (Monsignor) on Apr 23, 2005 at 13:48 UTC
-w turns on warning for the whole Perl program, including any included modules etc; 'use warnings' enables them only in the current lexical scope. Dave.	[reply]
Re: #!/usr/bin/perl -w and use warnings; by japhy (Canon) on Apr 23, 2005 at 13:48 UTC
The -w has a global effect on your entire program. The warnings pragma was introduced in Perl 5.6 and allows for scoped sections of warning-enabled code, with the ability to turn on and off specific classes of warning messages. _____________________________________________________ Jeff `japhy` Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and `perl` hacker How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart	[reply]
Re: #!/usr/bin/perl -w and use warnings; by tlm (Prior) on Apr 23, 2005 at 13:49 UTC
This is discussed in `warnings` and `perllexwarn`; from the former: 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. the lowliest monk	[reply]
Re^2: #!/usr/bin/perl -w and use warnings; by ghenry (Vicar) on Apr 23, 2005 at 13:51 UTC
Yes, I never thought of that, in that it would be useful to only have warnings for some sections, but the "-w" would be good to check other people modules. Do I understand that correctly? Walking the road to enlightenment... I found a penguin and a camel on the way..... Fancy a yourname@perl.me.uk? Just ask!!!	[reply]
Re^3: #!/usr/bin/perl -w and use warnings; by tlm (Prior) on Apr 23, 2005 at 14:55 UTC
The `-w` flag won't help you if the module's author explicitly turned warnings off. (I've run into this problem when a module's author turns off all warnings in a block, instead of selectively turning off a particular warning.) E.g. `% perl -wle 'use warnings; { no warnings; $x == 1 } print "ok"' ok % perl -Wle 'use warnings; { no warnings; $x == 1 } print "ok"' Useless use of numeric eq (==) in void context at -e line 1. Name "main::x" used only once: possible typo at -e line 1. Use of uninitialized value in numeric eq (==) at -e line 1. ok` [download] Update: Clarified the original reply. the lowliest monk	[reply] [d/l]
Re: #!/usr/bin/perl -w and use warnings; by ambrus (Abbot) on Apr 23, 2005 at 14:01 UTC
You've already got the correct answer. Just one more comment: the -w flag is equivalent to `BEGIN { $^W = 1; }`	[reply] [d/l]
Re: #!/usr/bin/perl -w and use warnings; by hubb0r (Pilgrim) on Apr 23, 2005 at 14:09 UTC
Plus, with the advent of the warnings pragma, you can use warnings for your entire program, but then locally scope a `no warnings; or no warnings qw(uninitialized);` [download] for a block of code where there is a good reason not to use warnings.	[reply] [d/l]
Re: #!/usr/bin/perl -w and use warnings; by brian_d_foy (Abbot) on Apr 23, 2005 at 17:06 UTC
Besides the other answers, `use warnings` implicitly requires perl 5.6 or later. People who have to support earlier versions of perl don't use the pragma. -- brian d foy <brian@stonehenge.com>	[reply] [d/l]
Re^2: #!/usr/bin/perl -w and use warnings; by ghenry (Vicar) on Apr 23, 2005 at 17:30 UTC
Have far back can you use the `-w` option? Just going to check the history files on perldoc. Walking the road to enlightenment... I found a penguin and a camel on the way..... Fancy a yourname@perl.me.uk? Just ask!!!	[reply] [d/l]
Re^3: #!/usr/bin/perl -w and use warnings; by brian_d_foy (Abbot) on Apr 23, 2005 at 21:34 UTC
Perl4 had the -w switch, and back then everything fit in one manpage (man perl) so that's where you need to look. You can still download perl4.036 from CPAN and just look for yourself. You don't have to actually install it :) -- brian d foy <brian@stonehenge.com>	[reply]
Re^4: #!/usr/bin/perl -w and use warnings; by ghenry (Vicar) on Apr 23, 2005 at 21:37 UTC
Re^3: #!/usr/bin/perl -w and use warnings; by Joost (Canon) on Apr 23, 2005 at 20:47 UTC
I think -w is usable for all perl 5 versions. Not sure about perl 4 though :-) "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply]
Re^2: #!/usr/bin/perl -w and use warnings; by ghenry (Vicar) on Apr 23, 2005 at 19:14 UTC
Was it introduced with Perl 5? I can't find any info on Perl 4, which was before my time. Walking the road to enlightenment... I found a penguin and a camel on the way..... Fancy a yourname@perl.me.uk? Just ask!!!	[reply]
Re^3: #!/usr/bin/perl -w and use warnings; by brian_d_foy (Abbot) on Apr 23, 2005 at 21:36 UTC
The -w switch was in Perl4. Even though it was before your time, Google still remembers it. :) -- brian d foy <brian@stonehenge.com>	[reply]
Re: #!/usr/bin/perl -w and use warnings; by gaal (Parson) on Apr 23, 2005 at 23:03 UTC
In addition to the above, there's the benefit described in Re: Perl Directive.	[reply]