You can't do something repeatedly without some loop.

That doesn't mean that all loops are equal, though. You might be able to use something like

$line =~ s/(?:\t|\z)\K/a/g;  # Requires 5.10+
$line =~ s/(\t|\z)/${1}a/g;  # Slower, but works with earlier versions
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Update: Accidentally said "with some loop." Fixed.