$ perl -le '
> @strs = ( q{abcd  }, q{abcdef}, q{      } );
> print qq{String: >$_< - },
>    m{(?=(\S)\s*$)}g
>    ? qq{found $1 at offset @{ [ pos() ] }}
>    : q{no match}
>    for @strs;'
String: >abcd  < - found d at offset 3
String: >abcdef< - found f at offset 5
String: >      < - no match
$