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

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Update: Clarified the original reply.

the lowliest monk

In reply to Re^3: #!/usr/bin/perl -w and use warnings; by tlm
in thread #!/usr/bin/perl -w and use warnings; by ghenry

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