Difference between 'use warnings' and '-w' switch

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

I need a bit of clarification on the differences between saying use warnings; and putting -w on the command line. I thought they were the same.

My code starts:

use strict;
use warnings;
use Net::FTP;
[download]

When I put -w on the command line, code within Net::FTP throws a warning at line 716 when trying to send a file. This is version 2.70 of Net::FTP and when I look at the source, it is trying to do -f $loc when $loc has never been assigned to (the warning is "uninitialised value in -f"). So the warning is genuine, nothing I can do to prevent it.

The reason behind the error is not really relevant here, but I'm wondering why if I just say "use warnings" without the -w switch, this warning is not reported?

Thanks.

s^^unp(;75N=&9I<V@`ack(u,^;s|$.+\`|"$`$'\"$&\"$"|ee;/m.+h/&&print$&

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Re: Difference between 'use warnings' and '-w' switch by dave_the_m (Monsignor) on Jan 11, 2006 at 12:23 UTC
use warnings is lexically scoped: it only enables warnings in the current block of code. -w is global: it enables warnings in all code, including Net::FTP. Dave.	[reply]
Re: Difference between 'use warnings' and '-w' switch by northwind (Hermit) on Jan 11, 2006 at 12:25 UTC
`-w` turns warnings on for the entire program, while `use warnings;` only turns on warnings for the current `/scope\|module\|file/`. Update:Meh, late on the post. First post describes it better (if better is defined as using Perl compliant language `:)` ).	[reply]
Re: Difference between 'use warnings' and '-w' switch by monkey_boy (Priest) on Jan 11, 2006 at 12:29 UTC
this is the full line from that module: `my $size = do { local $^W; -f $local && -s _ };` [download] note, he locally turns off the warnings , (old style -w/$^W warnings); This is not a Signature...	[reply] [d/l]
Re^2: Difference between 'use warnings' and '-w' switch by muntfish (Chaplain) on Jan 11, 2006 at 13:02 UTC
Thanks - actually the line in my version (2.70) is: `$ftp->_ALLO(-s _) if -f $loc;` [download] And since there is no code to initialise $loc before this line, it will always produce a warning. It looks like this has been fixed in later versions (I checked 2.75 from CPAN) - it has been changed to the line you describe (note $local not $loc). Thanks. s^^unp(;75N=&9I<V@`ack(u,^;s\|$.+\`\|"$`$'\"$&\"$"\|ee;/m.+h/&&print$&	[reply] [d/l] [select]