Update: For the complete story on this question, please see John's answer below. The subtlety is that perl's interpretation of the function/method call depends on whether or not the subroutine is defined when the call is compiled.

The syntax: 

ThatPackage::blah $foo

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is the method-object calling syntax. For instance, consider: 
my $baz = []; bless $baz, "ThatPackage";

ok( blah $baz, 'blah-baz');

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This will call ThatPackage::blah with first argument of $baz using object method dispatching. The object is expected to be a blessed reference (which is generally the case) or it can also be a package name. When the method is a fully qualified identifier (like ThatPackage::blah), perl will start looking in that specified package for the subroutine to execute (see the MyCritter example in man perlobj.) However, even in this case the object still must be either a blessed reference or an package name - although perl does not check to see if the package actually 'exists.' Since spaces are not allowed in package names you are getting an error when calling ThatPackage::blah $bar.

By contrast, the syntax: ThatPackage::blah($bar) is simple function calling syntax. In this case the subroutine ThatPackage::blah must be defined or an error will be raised.

When using method-object syntax I would say that verifying that the object at least looks like an object is a useful check to make sure that you're doing what you think you're doing.

In reply to Re: Sub calling sadness by pc88mxer
in thread Sub calling sadness by nefigah

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