The basic problem is that you're trying to give  ?: a statement where the operator expects an expression. Try something like these:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $count;
 F();
 ;;
 $count = 0;
 F();
 ;;
 sub F {
   defined $count ? do { print '=' until $count ++ == 5; }
                  : print 'Undefined count'
                  ;
   }
"
Undefined count
=
=
=
=
=

c:\@Work\Perl\monks>perl -wMstrict -le
"my $count;
 F();
 ;;
 $count = 0;
 F();
 ;;
 sub F {
   print defined $count ? '=' x 5 : 'Undefined count';
   }
"
Undefined count
=====
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The second example does not keep track in  $count of the number of  '=' characters printed and you may need this for some reason, but if you don't, the second example is the way I'd go.

Update: If you do want to keep track of the number of things printed, here's a variation on the second method:

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $count;
 F();
 dd $count;
 ;;
 $count = 0;
 F();
 dd $count;
 ;;
 sub F {
   print defined $count ? '=' x ($count += 5) : 'Undefined count';
   }
"
Undefined count
undef

=====
5
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