The solution I posted is a one-line perl command, to be run from a command shell prompt, not to be embedded within a perl source file.

If you're running under *nix, then you'll have to exchange the quotes like so:

perl -le'%h=(), undef @h{split"",$_}, keys %h == 3 and print for "0000
+".."9999"'
[download]

The following will run as a script:

#!/usr/bin/perl -lw
use strict;

for ( '0000'..'9999' ) {
    my %h;
    undef @h{split'',$_};
    keys %h == 3 and print;
}
[download]