If you don't know what type of file it is (commas or tabs) then you may have a problem because if you convert all the commas to tabs in a file you could end up converting legitimate commas to tabs. I'm not certain if the reverse would be true. (but I would assume so). But the conversion is pretty easy:

open(FILE,"+<tabs.txt") or die "Couldn't open tabs.txt: $!";
my @file = <FILE>; 
seek(FILE,0,0);
foreach(@file) {
     s/\t/,/g;
     print FILE $_;
}
close(FILE);
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Now for putting quotes around your values:

open(FILE,"+<tabs.txt") or die "Couldn't open tabs.txt: $!";
my @file = <FILE>; 
seek(FILE,0,0);
 foreach(@file) {
     chomp();
     my $line = "";
     my(@values) = split(/\,/,$_);
     foreach my $value(@values) {
          $value = "\"$value\"" if $value !~ /^\".*?\"$/;
          $line .= "$value,";
     }
     chop($line);   
     print FILE "$line\n";
} 
close(FILE);
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Hope that helps