my $userid = "rsennat";    # assume userid is set to a var

my $string = "$str1,$str2";

# NB: using double quotes so that the $userid is interpolated
# Single quotes would protect the literal string like \$userid
my $tmp_file = "/tmp/$userid";

open(TMPFILE, ">> $tmp_file") ||
  die "Can't write to '$tmp_file': $!\n";
print TMPFILE $string;
close(TMPFILE);