This uses a hash a bit like jethro's solution but it is populated by split'ing the tags part of the line after the timestamp and tags string have been captured. I use the hex representation of [ and ] in the regular expression to avoid confusing escaping. I've added another line to the log with the '48=' tag missing to show the change in output.

use strict;
use warnings;

my @wanted = qw{ 55 48 22 };

while ( <DATA> )
{
    my( $head, $tagStr ) = m{ (.*) \s+\x5b ([^\x5d]+) }x;
    print qq{Line      : $head\n};

    my %tags =
       map { split m{=} }
       split m{\x01}, $tagStr;
    print
       qq{Tags found:\n},
       map  { sprintf qq{            %-3s => %s\n}, $_, $tags{ $_ } }
       sort { $a <=> $b } keys %tags;

    print
       qq{Wanted    : },
       join q{|},
          map { exists $tags{ $_ } ? $tags{ $_ } : q{} } @wanted;
    print qq{\n===========\n};
}

__END__
2009/01/15 01:23:45:678: ASDF: [8=FIX.4.4
9=228
35=D
49=ZYXW
56=MYCO
3
+4=6
52=20090115-01:23:45
116=BLAH
129=HALB
50=MEH
1=HEM
11=abcefg1234
+56
15=ZZZ
21=1
22=5
38=100
40=2
44=4.80000000
48=ZVZZT.N
54=2
55=ZVZZ
+T
59=0
60=20090115-01:23:45
100=MEH
10=111
]
2009/01/15 01:27:09:154: QWER: [8=FIX.4.4
9=228
35=D
49=ZYXW
56=MYCO
3
+4=6
52=20090115-01:23:45
116=BLAH
129=HALB
50=MEH
1=HEM
11=abcefg1234
+56
15=ZZZ
21=1
22=5
38=100
40=2
44=4.80000000
54=2
55=ZVZZT
59=0
60=2
+0090115-01:23:45
100=MEH
10=111
]
[download]

The output.

Line      : 2009/01/15 01:23:45:678: ASDF:
Tags found:
            1   => HEM
            8   => FIX.4.4
            9   => 228
            10  => 111
            11  => abcefg123456
            15  => ZZZ
            21  => 1
            22  => 5
            34  => 6
            35  => D
            38  => 100
            40  => 2
            44  => 4.80000000
            48  => ZVZZT.N
            49  => ZYXW
            50  => MEH
            52  => 20090115-01:23:45
            54  => 2
            55  => ZVZZT
            56  => MYCO
            59  => 0
            60  => 20090115-01:23:45
            100 => MEH
            116 => BLAH
            129 => HALB
Wanted    : ZVZZT|ZVZZT.N|5
===========
Line      : 2009/01/15 01:27:09:154: QWER:
Tags found:
            1   => HEM
            8   => FIX.4.4
            9   => 228
            10  => 111
            11  => abcefg123456
            15  => ZZZ
            21  => 1
            22  => 5
            34  => 6
            35  => D
            38  => 100
            40  => 2
            44  => 4.80000000
            49  => ZYXW
            50  => MEH
            52  => 20090115-01:23:45
            54  => 2
            55  => ZVZZT
            56  => MYCO
            59  => 0
            60  => 20090115-01:23:45
            100 => MEH
            116 => BLAH
            129 => HALB
Wanted    : ZVZZT||5
===========
[download]

I hope this is of interest.

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