Presuming you're asking how to specify the name of the file? To do what you're doing here I would do:

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);
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You're better off opening a file and printing to it straight from Perl than relying on shelling out to echo or another command to do the same thing - shelling out (using system or *cough* backticks) uses another process, is less portable and takes away your script's full control over what you're doing.

The only minus with doing it this way is that you have 3 lines of code instead of 1, but I'd take that hit any day for the improvement it makes to the efficiency, security, reliability and portability of the program!