The  // defined-or operator was introduced in Perl version 5.10, which also introduced the  (?|pattern) "branch reset" extended pattern (see Extended Patterns in perlre), which can be used to similar effect.

use 5.010;  # need (?|pattern) "branch reset" regex construct

use strict;
use warnings;

my $result = <<'EOT';
transaction 1234567; promote; 2014/08/01 10:22:37 ; user: john
  # Performing a "promote" on file.
  /./abc/desf/test.pl 138699/1 (138700/1)

transaction 4578643; promote; 2014/08/01 10:22:37 ; user: sam
  /./abc/desf/test.pl 138699/1 (138700/1)
EOT

print qq{[[$result]] \n\n};

while ($result =~ m{
    transaction \s  (\d+) .*?
    user:       \s* (\S+) \s*
    (?| [#] \s* ([^\n]*) | ())  # comment or empty string to $3
    }xmsg) {
    my ($transaction, $user, $comment) = ($1, $2, $3);
    printf "%-12s %-10s %s\n", $transaction, $user, $comment;
    }
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Output:

c:\@Work\Perl\monks\sravs448>perl alt_branch_reset_1.pl
[[transaction 1234567; promote; 2014/08/01 10:22:37 ; user: john
  # Performing a "promote" on file.
  /./abc/desf/test.pl 138699/1 (138700/1)

transaction 4578643; promote; 2014/08/01 10:22:37 ; user: sam
  /./abc/desf/test.pl 138699/1 (138700/1)
]]

1234567      john       Performing a "promote" on file.
4578643      sam
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