Another difference is that if someone runs the perl command against your script then the shebang line will be ignored while "use warnings" will still be enforced.

It may be ignored by the shell, but it isn't ignored by perl.

foo.pl:

#!/usr/bin/perl -w 

$x = $y . $z;




$ ./foo.pl 
Name "main::y" used only once: possible typo at ./foo.pl line 3.
Name "main::z" used only once: possible typo at ./foo.pl line 3.
Name "main::x" used only once: possible typo at ./foo.pl line 3.
Use of uninitialized value in concatenation (.) or string at ./foo.pl 
+line 3.
Use of uninitialized value in concatenation (.) or string at ./foo.pl 
+line 3.

$ perl foo.pl 
Name "main::y" used only once: possible typo at foo.pl line 3.
Name "main::z" used only once: possible typo at foo.pl line 3.
Name "main::x" used only once: possible typo at foo.pl line 3.
Use of uninitialized value in concatenation (.) or string at foo.pl li
+ne 3.
Use of uninitialized value in concatenation (.) or string at foo.pl li
+ne 3.
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BTW, you can use perl -X to force it to ignore -w on the shebang, if you want.