I find it particularly useful when unpacking args that are passed as references. Then I can look at the subroutine and remember immediately to pass a reference rather than a list.

# Using two-letter notation...
sub foo {
  my( $something, $that_ar ) = @_;
  foreach my $item ( @{$that_ar} ) {...
}
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I know at a glance that this sub takes an array ref. If I didn't use some sort of notation, and if the sub's code were a little more complicated, it helps to clarify.

I'm in the habit of using (mostly) four-letter versions: _aref, _href, _fh, _sref. Perhaps there's value in shortening the identifiers by a couple of strokes (line length), but old habits die hard.