If you truly mean to have overlapping fields, I don't think unpack would work, but substr or a lookahead regex would:

substr: Note that second parameter is 0-based. A warning will be given if the start position is beyond (but not at!) the end of the string; the result will be shorter than the requested length if $in isn't long enough.

$one = substr($in, 23, 15);
$two = substr($in, 28, 10);
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Beware! If you use substr in an lvalue context, the warning gets promoted to an error:

$ perl -we'sub foo { print $_[0] } eval { foo(substr "abc", 4, 1); 1} 
+or die "croak: $@"'
croak: substr outside of string at -e line 1.
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regex: use one (?=) anchored at the beginning per field. The offsets are still 0-based.

$in =~ /^  (?=.{23}(.{15}))   # field one
           (?=.{28}(.{10}))   # field two
       /xs
   or warn "bad input: $in ";
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If columns may be shorter, use .{0,15} and .{0,10} or similar. If a starting column is beyond the end of the string, the regex will fail.