Because perl is successfully starting a process: the shell. It needs to start the shell to deal with the shell metacharacters, the quotes. Here's the explanation from perldoc perlopentut:

       What happens if you try to open a pipe to or from a non-existen
+t com-
       mand?  If possible, Perl will detect the failure and set $! as 
+usual.
       But if the command contains special shell characters, such as "
+>" or
       "*", called 'metacharacters', Perl does not execute the command
       directly.  Instead, Perl runs the shell, which then tries to ru
+n the
       command.  This means that it's the shell that gets the error in
+dica-
       tion.  In such a case, the "open" call will only indicate failu
+re if
       Perl can't even run the shell.  See "How can I capture STDERR f
+rom an
       external command?" in perlfaq8 to see how to cope with this.  T
+here's
       also an explanation in perlipc.
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"My two cents aren't worth a dime.";