my $x = 'st\x{f9}';
$x =~ s/\\x\{([\da-fA-F]{2,4})\}/pack("U",hex($1))/ge;

##</code><code>##

use Encode;

my $x = 'st\x{f9}';
my $iso = eval '"'.$x.'"';
$x = Encode::decode('iso-8859-1', $iso);