If you're talking about dynamically choosing the target of the link in response to another action on the page, you can't do it in perl. You can only target links on the client side: the server doesn't know anything about your frames or their relations. It just sends back the page it's asked for, and the browser takes care of displaying it in the right place. You'd need some javascript.

But just in case you're talking about the point where you return the page containing the link, as the $cust in your snippet suggests, then here's one of the approximately 17,354 ways to do it:

my $target = ($use_foo) ? 'foo' : 'bar';
print qq|<a href="..." target="$target"> $cust</a>|;
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you'll find many more in perlsyn.