The sort language construct accepts a code block argument for use as a custom comparison operator.

Example:

@list = sort {$b cmp $a} @list

The above code segment sorts @list in reverse to what is normally expected ('z', 'y', 'x', ..., 'a'). This can be extended in many ways -- one of which is to sort $msgid based on a value derived from $msgid:

@msgids = sort {
    $mailarray{$a}{'from'} cmp $mailarray{$b}{'from'}
}
    keys %mailarray;
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You should read up on 'sort' using 'perldoc -f sort'. Good luck.