Both of these answers ignored that the sub was an 'anonymous'/'inline' sub that would have access to surrounding local variables in the same scope, including regex vars.

I find it surprising that '$1' is affected in this way by an anonymous sub.

I wouldn't find it surprising that a normal sub would auto-save context to not completely disrupt callers (even though '$_' needs to be explicitly saved with local).

To re-ask, why is an inline-sub which I thought was designed to have access to local vars (in same context) restoring '$1'. If it was accessing or changing '$_1', it would access the copy of the sub it was in. I had supposed that the '$1' would stay constant until another regex and that an inline/anon sub wouldn't treat '$1' differently from '$_1'.

I was really more wondering what the rational might be for treating them differently in an anon/inline sub.

In the same way I find that '$1', and '$2' are cleared coming out of a 'do' block to be strange -- I would have thought only another regex would change them.

my $s="abcdefg";
$_=$s;
my @res=do { m{abc(de)(fg)}; };
P "nres=%s 1=%s, 2=%s", 0+@res, $1, $2;
'
nres=2 1=∄, 2=∄