my $log = 0;
print "Do you want to log to a file?";
my $resp = <STDIN>;
if ($resp =~ /^y/i) {
   $log = IO::Tee -> new(">stdout.txt", \*STDOUT);
}
...
if $log {
   print $log "What did I want to say?\n"
} else {
   print "What did I want to say?\n"
}