Another example of the effect of what you were doing in the OP (i.e., a non-zero-width capture within a zero-width assertion):

Win8 Strawberry 5.8.9.5 (32)  Wed 11/11/2020 18:02:18
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings -l
my $s = 'ABCDE';

while ($s =~ m{ (?= (\w+)) }xmsg) { print $1; };

^Z
ABCDE
BCDE
CDE
DE
E
[download]

Update: This might be a better example of overlapping capture:

Win8 Strawberry 5.8.9.5 (32)  Wed 11/11/2020 21:41:12
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings -l
my $s = 'ABCDE X YZ FGHI';

while ($s =~ m{ (?= (\w{3})) }xmsg) { print "'$1'"; };

^Z
'ABC'
'BCD'
'CDE'
'FGH'
'GHI'
[download]