Considering that one can have a file Abc.pm 
sub Abc::foo { ... }
sub Abc::bar { ... }

[download]
without any package statement, or even: 
BEGIN {
    my $package = ... Complicated expression ...;
    eval <<"EOT"
package ${package};
sub foo { ... }
sub bar { ... }
EOT
}

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I think you can forget about catching all possible cases.

Considering that the majority of the people will write package Abc; near the top of the file, and all you seem to want is earlier program termination, why not perform a check that will catch the majority of the error, and let the few cases where 1) the author doesn't use "package Abc", and 2) the author miscapitalizes the package name, and 3) the author uses a case insensitive file system die sometime later (due to missing subroutines)?

In reply to Re^3: Finding out whether a module contains a certain package by JavaFan
in thread Finding out whether a module contains a certain package by rovf

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