Create two pieces of code, test1.pl and blah.pm:

use warnings;

use blah;

sprintf("%d\n", 1); #this is always warned, as it is in a scope with "
+use warnings" declared
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And here is blah.pm:

package blah;

sprintf("%d\n;", 1); 
1;
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Run perl test1.pl, you get:

Useless use of a constant in void context at test1.pl line 5.
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With perl -w test1.pl, you get:

Useless use of a constant in void context at blah.pm line 3.
Useless use of a constant in void context at test1.pl line 5.
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