I find interpolating/concatanating of any complexity mainly looks messy. Try to add dereferencing hash/array refs or method calls to the mix and it gets worse. I get on better with a template so I often press sprintf into service.

$hotfix = sprintf(
  q{%s_%s_%s_%s.zip},
  $app,
  $mod,
  $bld,
  $hf,
);
[download]

Expressions, ternaries, refs, method calls, etc. will fit nicely while still, imo, being readable.

Although in the second (simple) case

$fullpath = join(q{/}, $dir, $filename);
[download]

looks better to me.

ymmv