chdir to your Perl's lib directory and search for *heavy*. The idea is to not immediately load things which take (small, but sometimes not insignificant) time to load if you are likely not going to use them. "require Exporter" now delays loading most of the code that used to be in Exporter.pm until such time as someone uses an "advanced" feature of Exporter.pm. At that time it loads some "Exporter*heavy*" thingy (there is some variability in the details of how each of these is done, but "heavy" seems a common component). Other heavily used modules do similar tricks.

At some point, at least one module author decided to delay loading Carp.pm until such time as a need for it was demonstrated. I'm not sure how good the evidence was that motivated this decision, but the idea was followed by other module authors under the assumption that there was good enough reason for one to go through this small bit of extra work.

In any case, I do it in some of my modules out of what some would call "cargo cult" practice and some would call "best practices", depending on whether they want to make such look bad or good.

But I'm well aware of the pitfalls of conditional compile-time magic and so don't leave off my parens. Actually, my soon-to-be-released module does:

sub _croak {
    require Carp;
    return \&Carp::croak;
}

sub Whatever {
    _croak->( "Usage: Whatever()" )
        if  0 < @ARGV;
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(because I don't like depending on the sometimes-inappropriate "intelligence" Carp tries to apply to skipping stack frames to be reported)