my $ok = 1;       # set private variable $ok = 1
$str =~ m/(.)(.)(??{ $ok = 0 if $2 lt $1; 0 })/;

##</code><code>##

my $ok = 1;
local *myok = \$ok;    # Needed to avoid closure in code assertion.
$str =~ m/(.)(.)(??{ $myok = 0 if $2 lt $1; 0 })/;