POD to the rescue

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perldoc -f printf

    printf FILEHANDLE FORMAT, LIST
    printf FORMAT, LIST
            Equivalent to "print FILEHANDLE sprintf(FORMAT, LIST)", ex
+cept
            that "$\" (the output record separator) is not appended. T
+he
            first argument of the list will be interpreted as the "pri
+ntf"
            format. If "use locale" is in effect, the character used f
+or the
            decimal point in formatted real numbers is affected by the
            LC_NUMERIC locale. See the perllocale manpage.

            Don't fall into the trap of using a "printf" when a simple
            "print" would do. The "print" is more efficient and less e
+rror
            prone.


perldoc perllocale 

Category LC_NUMERIC: Numeric Formatting
In the scope of use locale, Perl obeys the LC_NUMERIC locale informati
+on, which controls an application's idea of how numbers should be for
+matted for human readability by the printf(), sprintf(), and write() 
+functions. String-to-numeric conversion by the POSIX::strtod() functi
+on is also affected. In most implementations the only effect is to ch
+ange the character used for the decimal point--perhaps from '.' to ',
+'. These functions aren't aware of such niceties as thousands separat
+ion and so on. (See The localeconv function if you care about these t
+hings.)

Output produced by print() is also affected by the current locale: it 
+depends on whether use locale or no locale is in effect, and correspo
+nds to what you'd get from printf() in the ``C'' locale. The same is 
+true for Perl's internal conversions between numeric and string forma
+ts:

        use POSIX qw(strtod);
        use locale;
        $n = 5/2;   # Assign numeric 2.5 to $n
        $a = " $n"; # Locale-dependent conversion to string
        print "half five is $n\n";       # Locale-dependent output
        printf "half five is %g\n", $n;  # Locale-dependent output
        print "DECIMAL POINT IS COMMA\n"
            if $n == (strtod("2,5"))[0]; # Locale-dependent conversion
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___crazyinsomniac_______________________________________
Disclaimer: Don't blame. It came from inside the void
perl -e "$q=$_;map({chr unpack qq;H*;,$_}split(q;;,q*H*));print;$q/$q;"

Perhaps a locale setting from one of the POSIX:: modules might help? I haven't used locale stuff but it may do exactly the desired thing.

Also you could use sprintf instead of printf and then fix up the result. Like:

#(other program stuff here)

my $a=10.5;
my $str=sprintf "%.1f",$a;
$str =~ s/\.(\d)$/,$1/; #only do the last . and preserve the digit
print $str;

#(and more program stuff here)
[download]

Of course some of the lines could be collapsed together to make it less lines of code.

Use a custom format sub such as the following I wrote:

my $num = 244.83;
print "'" . str($num, 8, 3) . "'\n";

sub str {
   ## This sub is used to return a formatted numeric string.
   ## It has three arguments which are: the number sent, the
   ## desired width (including decimal point, if any), and
   ## the number of decimal places (optional) in that order.
   ## The number is right-justified.
   my ($sentnum, $width, $precision) = @ARG;
   ## Check to see if a non-zero precision was sent.
   if ($precision) {
      ## print to var using decimal point.
      $sentnum = sprintf("%" . "$width.$precision" . "f", $sentnum);
   } else {
      ## print in integer format.
      $sentnum = sprintf("%" . "$width.0" . "f", $sentnum);
   }
   ## change decimals to commas
   $sentnum =~ tr/./,/;
   return $sentnum;
}
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Update: could move tr/./,/ to above else statement.
Results: ' 244,830'