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
=====
[download]

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
[download]

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.