If your program contains:

require "foo.pl";
&do_this("with that");
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and foo.pl contains:

sub do_this { print @_; }
1;
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Then this code must pass -c, because there's syntactically nothing wrong with it, and it cannot detect that foo.pl defines do_this, because that's not pulled in during compile time (it doesn't even need to exist!).

Finding a subroutine doesn't start until it gets invoked. This has long been the way, necessarily.

-- Randal L. Schwartz, Perl hacker