open(FOO,">/temp/foo") || die $!;
sub foo { local *STDOUT=*FOO; print "foo"; } #yes, this is supposedly to STDOUT
foo;
close(FOO);


##</code><code>##

*FOO=*STDOUT; #Save it somehow
open(STDOUT,">>outputfile") || die $!;
sub foo { print "foo bar"; }
foo;
close(STDOUT);
*STDOUT=*FOO;