This may or may not be an improvement, but it only takes one pass through the regex: 
#!/usr/bin/perl

use strict;
use warnings;

# If all four chars are present, returns them in first-seen order.
# If any of the four are not present, returns an empty string.
sub four_or_nothing {
    my $str = shift;

    my @in_order = ($str =~ m{ 
        ([abcd]) 
        .*?
        (?!\1)([abcd]) 
        .*?
        (?!\1|\2)([abcd]) 
        .*? 
        (?!\1|\2|\3)([abcd])
    }x);

    return join("", @in_order);
}

foreach my $case (
    '1b22a7b3d3ccaacbcaccddeqcc11oabepd',
    '1b22w7b3d3ccsqcbc2ccddeqcc11o9bepd',
) {
    printf qq{%s: "%s"\n}, $case, four_or_nothing($case);
}

__DATA__
1b22a7b3d3ccaacbcaccddeqcc11oabepd: "badc"
1b22w7b3d3ccsqcbc2ccddeqcc11o9bepd: ""

[download]

In reply to Re^4: How to replace greedy alternation? by Anonymous Monk
in thread How to replace greedy alternation? by adamcrussell

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