Here's another version without the @-

Just match any four of the digits (from an ordered string). 

#!/usr/bin/perl

# https://perlmonks.org/?node_id=1217042

use strict;
use warnings;

'0123456789' =~ /(.).*?(.).*?(.).*?(.)(?{print "$1$2$3$4\n"})(*FAIL)/;

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In reply to Re^3: list of four digit lock combinations without repeated digits by tybalt89
in thread list of four digit lock combinations without repeated digits by Lotus1

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