If I'd thought ahead I'd have used a lookahead like this:

use warnings;
use strict;

while (<DATA>) {
    chomp;
    next if ! /^
        (\d+)\s # Capture the number
            (?: # Start of non-capture group
                MatchMe\s\[(?=.*\]$) # Match matchme [.*]
                |
                I\sneed\sto\sbe\sdispatched.*\:\s(?=[^\]]*$)
            )
        (.*?)\]?$/x; #capture reason

    print "Line $1, reason: $2\n";
}
__DATA__
111 MatchMe [This is why]
222 I need to be dispatched to the same callback: For This Reason
112 MatchMe [This is why
223 I need to be dispatched to the same callback: For This Reason]
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OP should note the positive lookahead asertions (?=...) that prevent the false matches found by the simpler expression. Prints:

Line 111, reason: This is why
Line 222, reason: For This Reason
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