If the sub isn't declared or defined before use, then you can't call it like a builtin - without & and parentheses around the arguments. That's the main disadvantage for me.
Huh?!? You're joking? Indeed I do declare my subs to be able to call them without parentheses - fair enough. But it is never stressed enough that in Perl5 (as opposed to Perl4 and previous versions) the &-form of of sub call should never ever be used unless you really need it, i.e. you do know what you're doing. See perlsub.

In reply to Re^3: Warning for "unused sub declarations"? by blazar
in thread Warning for "unused sub declarations"? by blazar

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