It's usually best/simplest to just filter out the bad results.

my @x;
while (/(?=((\d)(\d)))/g) {
   push @x, $1 if $3 > $2;
}
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You could also do that nicely using a lookup.

my %ok;
for my $i (1..8) {
   for my $j ($i+1..9) {
      $ok{"$i$j"} = 1;
   }
}

my @x = grep $ok{$_}, /(?=(\d\d))/g;
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You could build a complex pattern.

use Regexp::List qw( );

my @ok;
for my $i (1..8) {
   for my $j ($i+1..9) {
      push @ok, "$i$j";
   }
}

my $re = Regexp::List->new()->list2re(@ok);
my @x = /(?=($re))/g;
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In can be done programmatically in the regexp as well.

my @x;
push @x, $1 while /(?=((\d)(\d)(?(?{ $3 <= $2 })(?!))))/g;
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(It's pretty crazy that I was able to write the last one without errors and without checking perlre, so you probably shouldn't use that one.)

All tested.