Are you sure there is anything in $content? One way to check would be to use the debugger.

Update: as a general thing, simultaneous read and write access to a file gets messy. Also, not sure, but your use of DATA as the file handle might be causing trouble as well; it's a special word.

use strict;
use warnings;

open my $fh,'>>','logname' or die "Could not open logname";
while (1) {
  my $content = $rh->cmd(...);
  if (defined $content) {
    print $fh $content
  } else {
    print 'Nothing to be done!'
  }
  sleep 1;
}
close $fh;
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