I was referring to skip "rm -rf cover_db"

Okay, that comes from my distro's Makefile.PL lines 124-129:

# addition to realclean: also delete coverage database, if testcover w
+as run
realclean ::
    $(NOECHO) ( $(TEST_D) cover_db && $(RM_RF) cover_db ) || $(ECHO) r
+ealclean:: skip "rm -rf cover_db"
    $(NOECHO) ( $(TEST_F) LICENSE.pod && $(RM_RF) LICENSE.pod ) || $(E
+CHO) realclean:: LICENSE.pod intermediary
    $(NOECHO) ( $(TEST_F) README.pod && $(RM_RF) README.pod ) || $(ECH
+O) realclean:: README.pod intermediary
    $(NOECHO) ( $(TEST_F) MYMETA.json.lock && $(RM_RF) MYMETA.json.loc
+k ) || $(ECHO) realclean:: MYMETA.json.lock temp file
[download]

It is using a variety of macros:

• the $(NOECHO) macro from MakeMaker, which on cmd.exe prefixes the command with "@" so it won't print the command line first.
• my $(TEST_D) macro, defined on lines 109-110, which is inspired by the MakeMaker macro $(TEST_F) for testing the existing of a file. When I looked at the Makefile for $(TEST_F) implementation on my system, I saw it used ExtUtils::Command from the ExtUtils::MakeMaker distro, which as described makes versions of common UNIX commands from makefiles, and makes them available to the Makefile, for doing a common linux/bash idiom in non-linux/non-bash shells. I took that inspiration for the $(TEST_D) macro, which tests for directory existence using the same module/idiom
• the $(RM_RF) is a MakeMaker-defined macro, which again can use ExtUtils::Command for its implementation on systems that don't know rm -rf, and may just expand to rm -rf on systems that do know it
• The || works on both Windows and Linux for only running the second command if the first command fails, and && for only running the second if the first passes. I don't know how portable it is beyond those (I have never run a rule with the || or && syntax on any system other than Windows or Linux -- I do my development mostly on Windows with occasionally running things on Linux, so that's where I run my custom makefile targets; CI and the CPAN smoketesters run my Makefiles on a variety of other systems, but those just run the standard targets for the make/test sequence).
• $(ECHO) resolves to a ExtUtils::Command call on my system.

By using the macro variables that MakeMaker defines in the Makefile instead of literally using common UNIX-style commands, you can write rules that are much less dependent upon the system you are on, the availability of bash, etc., and will work cross-platform but will still look and behave like the linux commands that many are used to using.

Again, I recommend looking through the Makefile that perl Makefile.PL generates in your build environment; using that, you can see what macros are available to you, and how they are implemented on your system.