use strict;
use warnings;
# Note: doesn't handle newlines in quoted values

my $out = $ARGV[0].".tab";
open OUT,">$out" or die "Can't open output $out\n";
while (<>) {
   my $tab = "";
   my $qv=0; # Quoted value indicator
   my $dq=0; # Double quote flag indicates the previous character was a "
   for (split //) {
      # Start of a quoted value
      if (not $qv and $_ eq '"') { $qv=1; next; }
      
      # Double quotes within or at the end of a quoted value
      if ($qv and $_ eq '"') { $dq=1; next; }

      # If last char was a double quotes OR we're not within a quoted value, comma = tab
      if (($dq or not $qv) and $_ eq ',' ) { $dq=0; $qv=0; $_="\t"; } # End of field

      # Two consecutive double-quote characters within a quoted value
      elsif ($dq and $_ eq '"') { $dq=0; } # Double double quotes
      $tab .= $_;
   }
   print OUT $tab;
}