If you change use a lexical variable for for SAVE_OUT then the warning goes away. It is also safer, you never know when there will be another SAVE_OUT somewhere else in the code and data mysteriously disappears.

open( my $SAVE_OUT, ">&", STDOUT );
open( STDOUT, "> $POST_FILE" ) || die "Can't redirect stdout";

#more code goes here ...
#end of the subroutine:
#need to restore STDOUT(!?)
close(STDOUT);
open( STDOUT, ">&", $SAVE_OUT );
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Update: There are some fast monks out there;)