...and then use open on that scalar to reopen the STDxxx filehandles.

Irrespective of whether your actual issue with open and the tied scalar can be solved (which I believe isn't possible), there's another problem you'd most likely run into, if the idea behind this is to capture stdout/stderr of external programs run via system().

There are two types of file handles in Perl: internal ones, and those associated with a system file descriptor (you can tell them apart with fileno(), which returns -1 for internal file handles). The thing is that handles you've opened to a scalar ref are always internal, i.e. without a system file descriptor. But when system() fork/execs, the child will only inherit those system file descriptors, not Perl internal file handles...  In other words, the child's output to stdout/stderr (typically file descriptor 1/2) would never go to the Perl scalar you've opened your handle to.

# this would try to re-use STDERR's file descriptor 2,
# which dies with a "Bad file descriptor" error
open STDERR, ">", \$variable or die $!;

# closing STDERR first 'detaches' STDERR from file descriptor 2
# so it is then being re-opened to an internal file handle
printf "fd=%d\n", fileno(STDERR);   # 2
close STDERR;
open STDERR, ">", \$variable or die $!;
printf "fd=%d\n", fileno(STDERR);   # -1  (not accessible from system(
+)'s child process!)
[download]

Update: also see capturing stderr of a command, invoked via backticks.