This version directly generates the list of desired patterns rather than filtering through all 4-digit patterns, just to demonstrate another approach. It also works for any length of digit string. 

#/usr/bin/perl -w
use strict;
use Algorithm::Loops qw< NestedLoops NextPermute >;
my( $uniq )= ( @ARGV, 3 ); # 3 unique digits plus one duplicate digit
my $digs= NestedLoops(
    [ [0..9], ( sub { [1+$_..9] } ) x ($uniq-1) ],
);
my @digs;
while(  @digs= $digs->()  ) {
    do {
        for my $dup (  0 .. $#digs  ) {
            for my $after (  $dup .. $#digs  ) {
                my @result= @digs;
                splice @result, 1+$after, 0, $digs[$dup];
                print @result, $/;
            }
        }
    } while(  NextPermute( @digs )  );
}

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- tye        

In reply to Re^2: Find duplicate digits (non-filter) by tye
in thread Find duplicate digits by Anonymous Monk

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