There is a workaround:

#!/usr/bin/perl -w
$x = "123 56"; 
$x =~ / /g;
print "$x, ", pos($x), "\n";
{ local *x }                     # <---
print "$x, ", pos($x), "\n";
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The catch: @x, %x, &x, etc will get localized too. In other words, if you use local *_ to protect the pos of $_, you will lose access to the current @_ for the remainder of block.