I would code it like this:

#!/usr/bin/perl -w

use strict;
use Fcntl ':flock';

sub add {
  open FH, ">>$file" or die "Cant open config file $!\n";
  flock(FH,LOCK_EX);
  print FH "zone \"$formdata{domain}\" in \{\n\ttype slave\;\n\tfile\"
+$client\/db.$formdata{domain}\"\;\n\tallow-query \{ any\; \}\; \}\;\n
+\n";
  close FH;
}
[download]

I see very little advantage in 1 line subs - the lock sub is three lines where 1 will do. If you wanted to make a more useful lock sub that included a timeout/retry feature you would need to pass the file handle to the sub. May as well pass the lock type too:

#!/usr/bin/perl -w

use strict;
use Fcntl ':flock';

sub add {
  open FH, ">>$file" or die "Cant open config file $!\n";
  lock_file(*FH, LOCK_EX);
  print FH "zone \"$formdata{domain}\" in \{\n\ttype slave\;\n\tfile\"
+$client\/db.$formdata{domain}\"\;\n\tallow-query \{ any\; \}\; \}\;\n
+\n";
  close FH;
}

sub lock_file {
   local *FH = shift;
   my $lock = shift;
   my $count = 0;
   my $timeout = 15;
   until (flock FH, $lock) {
       sleep 1;
       die "Can't get lock\n" if ++$count > $timeout;
   }
}
[download]

When you close a file you automatically release any locks so you can just drop the whole concept of unlocking provided you explicitly close.

cheers

tachyon

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