perlvar says "read-only and dynamically scoped"

They are. mysub does indeed back them up on entry and restore them on exit.

$ perl -E'
   sub f { say $1; "b"=~/(b)/; say $1; }
   "a"=~/(a)/;
   say $1; f(); say $1;
'
a
a
b
a
[download]

The problem is that $_[0] and $_[1] are aliased to $1 and $2, so changing $1 and $2 changes $_[0] and $_[1].

$ perl -E'
   sub f { say $_[0]; "b"=~/(b)/; say $_[0]; }
   "a"=~/(a)/; f($1);
'
a
b
[download]

Three solutions:

• /.../;
  mysub("$1", "$2");
  sub mysub {
     /.../;
     shift;
     shift;
  }
  [download]

• my ($x, $y) =~ /.../;
  mysub($x, $y);
  sub mysub {
     /.../;
     shift;
     shift;
  }
  [download]

• /.../;
  mysub($1, $2);
  sub mysub {
     my ($x, $y) = @_;
     /.../;
     $x;
     $y;
  }
  [download]