my $string = <<'STRING';
@article{ 
        author = {Author, J.P } ,
        title  = "A paper, about things" ,
        etc..
     }
STRING
my $open = '';
my $commaless = $1 if $string =~ s/^(\s*@\w+\s*{)//;
for (split //,$string) {
    if (/{|"/ and not $open) {
        $open = /{/ ? '}' : '"';
        $commaless .= $_;
      next;
    }
    $commaless .= $_ unless /,/ and $open;
    $open = '' if $open eq $_;
}
print $commaless;