Here are a couple of nested loop solutions that are probably what the *::Combinatorics modules do under the hood.

johngg@abouriou ~/perl/Monks $ perl -Mstrict -Mwarnings -E '
for my $w ( 0 .. 6 )
{
    for my $x ( $w + 1 .. 7 )
    {
        for my $y ( $x + 1 .. 8 )
        {
            for my $z ( $y + 1 .. 9 )
            {
                say join q{}, $w, $x, $y, $z;
            }
        }
    }
}' | wc -l
210
[download]

johngg@abouriou ~/perl/Monks $ perl -Mstrict -Mwarnings -E '
say for
   map {
       my $w = $_;
       my $x = $w + 1;
       map {
           my $y = $_ + 1;
           map {
               my $z = $_ + 1;
               map {
                   join q{}, $w, $x, $y, $z
               } $z .. 9
           } $y .. 8
       } $x .. 7
    } 0 .. 6;' | wc -l
210
[download]

Done just for fun, the modules are probably the best way to go.

Update: Ignore the map version as it is giving incorrect results, right number but wrong digits.

Update 2: Spotted my error in the map version, I was initialising the three inner variables too early. This works:-

johngg@abouriou ~/perl/Monks $ perl -Mstrict -Mwarnings -E '
say for
   map {
       my $w = $_;
       map {
           my $x = $_;
           map {
               my $y = $_;
               map {
                   my $z = $_;
                   join q{}, $w, $x, $y, $z;
               } ( $y + 1 ) .. 9
           } ( $x + 1 ) .. 8
       } ( $w + 1 ) .. 7
   } 0 .. 6;' | wc -l
210
[download]