You could do it this way: (Basically merging the two regexes so that only two sets of capture brackets appear.)

#! perl -slw
use strict;

my @lines = (
'111 MatchMe [This is why]',
'222 I need to be dispatched to the same callback: For This Reason',
);

for ( @lines ) {
    m/
        ^(\d+) \s
        (?:
            (?-x:I need to be dispatched.*?: )
        |
            (?-x:MatchMe \[)
        )
        (.*?)
        \]?
        $
    /x and print "'$1'$2'";
}
__END__
C:\test>junk
'111'This is why'
'222'For This Reason'
[download]

Though you will have to decide whether the possibility of obtaining a false matche against lines like update: these

111 MatchMe [This is why
222 I need to be dispatched to the same callback: For This Reason]
[download]

is a problem for your application?