You could do

use lib 'modules_src';
use MyPackage;
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or

BEGIN {
    require 'modules_src/MyPackage.pm';
    # (call import() here, if required...)
}
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(assuming your current working directory is set such that it contains modules_src; otherwise specify the absolute path)

Though, personally, I prefer to stick to the convention, i.e. if your module is named Sth::MyPackage, then put it in a subdirectory named accordingly: modules_src/Sth/MyPackage.pm. Then, you can write

use lib 'modules_src';
use Sth::MyPackage;
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and have your use statement properly indicate the module's namespace...