What could cause Perl system calls to start failing?
It turned out the reason of this weird behavior was incorrectly defined PERL5SHELL variable: cmd.exe (the shell interpreter in Windows) should be called with some parameters for proper processing - there parameters went missing after some updates. )
By the way, in The Doc it's said that Perl usually assumes the 'cmd.exe /x /c' line as a shell executable anyway if PERL5SHELL environment variable is not defined at all.
P.S. I really like this thread: it clearly shows the purpose of comments. )