Rather than using substr, eval might be a more straightforward way (although admittedly somewhat hackish by the standard of the OP) to pull out the contents of substitution capture groups to either an array or a set of lexical variables. The die statement serves as a (perhaps half-assed) check that the number of captures is as expected.

>perl -wMstrict -le
"sub do_something_with {
   print q{in do_something_with: '}, join(q{' '}, @_), q{'};
   return join '', reverse @_;
   }
 my $s = 'abc ABC abc';
 $s =~ s{ (a)(b)(c) }
        { die qq{wrong number ($#-) of captures} if $#- != 3;
          my ($aye, $bee, $cee) =
            map eval qq{\$$_}, 1 .. $#-;
          do_something_with($aye, $bee, $cee);
        }xmsige;
 print $s;
"
in do_something_with: 'a' 'b' 'c'
in do_something_with: 'A' 'B' 'C'
in do_something_with: 'a' 'b' 'c'
cba CBA cba
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Note that every capture group defined in the regex is always represented in  @- even if it does not actually match anything. See what happens if the regex is changed to
    $s =~ s{ (a)(b)(c)(d)? }{ ... }xmsige;
with appropriate adjustment of the number against which  $#- is checked in the die statement.