The workaround seems fine -- it's straightforward and legible. You could probably make sure that $foo is never undef, too, so you don't have to worry about it. Something like:

  my $foo = getFoo() || '';
  if ($foo =~ /bar/ ){ ... }
[download]

But this has limitations (like if getFoo() returns 0).

side note #1 -- might just be your example, but treating foo as a boolean instead of string matching for "true" might work better, and avoids the warning. something like (again, depends on what getFoo() can return):

  my $foo = getFoo() ? 1 : 0;
  if ( $foo ){ ... }
[download]

side note #2 -- I'm sure someone will mention it if it's true, but i think perl6 has some new operators that can handle things like that (but i might be mis-remembering).
Update: I was thinking of (found it here) this:

$a // $b;   # short for:  defined($a) ?? $a :: $b
$pi //= 3;
# so could do (of course perl6 only):
if( $foo // '' eq "true" ){ ... }
# or
my $foo = getFoo();
$foo //= '';
if( $foo eq "true" ){ ... }
[download]