Using a ramfile (or maybe IO::String for pre-5.8) for your buffering saves a lot of complication. This only requires two methods and 10 lines of code:

#! perl -sw
package IO::Delayed;

sub new {
    my $class = shift;
    ## Create an anonymous glob
    my $self = do{ local *GLOB; \*GLOB };
    
    ## Use it to hold both the both the filehandle 
    ## and a ramfile
    open *$self, '>', \${ *$self } or die $!;
    
    ## bless and return it
    return bless $self, $class;
}

sub setPath {
    my( $self, $path ) = @_;
    
    ## Re-use the glob for the real file
    open $self, '>', $path or die $!;
    
    ## Output the buffered data
    print $self ${ *$self };
    
    ## Free up the storage
    undef ${ *$self };
    
    return;
}

return 1 if caller;
package main;

my $log = IO::Delayed->new; ## create a rambuffered filehandle

print $log "$_\n" for 1 .. 10; ## log some stuff to it

$log->setPath( 'mylog.out' ); ## Now redirect it to disk

print $log "$_\n" for 100 .. 110; ## and it becomes a normal filehandl
+e

__END__
C:\test>junk4

C:\test>type mylog.out
1
2
3
4
5
6
7
8
9
10
100
101
102
103
104
105
106
107
108
109
110
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