If that matters to you then you just save a copy of the variable's pos value and restore it after stomping on it. This is partly why it's writable.

$var = 'the quick brown fox';

while( $var =~ m/([aeiou])/g ) {
   # Save pos
   $pos = pos $var;

   # Do something to stomp on $var's pos value

   # Restore pos
   pos($var) = $pos;
}
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