Can someone explain this spec file?

[simon@bp]

I am investigating creating some rpm packages for Perl packages that are not included in the SCLO (for example XML::LibXML), so I downloaded and extracted the spec file for an existing package to use as an example:
rh-perl524-perl-Archive-Tar-2.08-5.el7.src.rpm

The spec file has this on the top line:
%{?scl:%scl_package perl-Archive-Tar}

Can someone explain this line to me?

[sml]

The macro %scl_package is defined in /etc/rpm/macros.scl (or /usr/lib/rpm/macros.d/macros.scl on CentOS 8) which is installed by scl-utils-build. That means a spec file using it BuildRequires scl-utils-build.

As to its semantics, see SCL Guidelines. Another document to consult is Software Collections Guide.

spec2scl helps with conversion.