Why would anyone -- 15 years ago, or today -- try to run the Strawberry Perl uninstaller, see a message that says "Another application has exclusive access to the file NTUSER.DAT Please shut down all other applications, then click Retry", and assume the problem was with Strawberry Perl and/or its uninstaller? The message literally says "another application".

If some other application is actively editing NTUSER.DAT (ie, the registry), then the Strawberry Perl uninstaller won't be able to. One could use something like the Microsoft's SysInternals ProcessExplorer to Find > Find Handle or DLL and search for NTUSER.DAT, and it will tell you every exe or dll that is currently accessing that file. That will tell you what might be interfering with the uninstaller's ability to remove the Strawberry Perl entries from the registry. (I use Process Explorer quite frequently for such things, like whenever I try to delete a file and Windows tells me that some application has it open.)

After running the search in Process Explorer, if and only if it told me that only the uninstaller was accessing NTUSER.DAT, then I would start looking into why something might be wrong on the uninstaller side. But even then, I wouldn't blame "perl", I would blame the uninstaller. But it's most likely, as it says, "another application" that is causing the problem.