archivers/upx: Fix build due to missing perl (pod2*) tools

upx unconditionally runs its docs target, which relies on multiple [1] pod2*
tools provided by perl.

USES=perl and USE_PERL=build were removed in ports r478489, resulting in those
tools not being available at build time, and the following errors:

  /bin/sh: pod2man: not found
  /bin/sh: pod2html: not found
  /bin/sh: pod2text: not found

This change, accordingly, re-adds perl as an unconditional build dependency,
including a comment about what its for.

While I'm here:

  * Make COMMENT portlint compliant [2]
  
[1] pod2man, pod2html, pod2text
[2] Wikipedia, MajorGeeks and others aslso describe UPX without a "The" prefix

PR: 234002
Reported by: <mail ozzmosis com>
Approved by: portmgr (blanket: build fix, JFI)
Differential_Revision: D18549
MFH: 2018Q4