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