If, as it seems, you want to get a match on a line which matches the specified UFOFH ... pattern, and print the whole line (or dump it into a variable), I would do the following (and, of course, TMTOWTDI):

$mailbox =~ m#UFOFH( [\d/]{5}){4,27}# && print $&;
[download]

This way, you use the $& variable to print out whatever was matched. With this implementation, any additional characters after 4 to 27 5-character blocks will be silently discarded, as will any characters before the UFOFH. You can prevent this (if you want) by rewriting the code as:

$mailbox =~ m#^UFOFH( [\d/]{5}){4,27}$# && print $&;
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to trap the beginning and end of the string. Don't forget to chomp() it though!

LAI
:eof