if (@vars= /(\s)(a)(\s)/ig){
    ...
}
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Another approach to handling overlapping matches (and, IMHO, better because less error-prone and better integrated into the general regex expression than the manipulation of pos) is the use of a look-ahead assertion to wrap capturing groups: see  "(?=pattern)" in the Look-Around Assertions sub-section of perlre Extended Patterns. Replacing
    /(\s)(a)(\s)/ig
with
    /(?=(\s)(a)(\s))/ig
in the code quoted above from the OP produces the desired capture (tested).