while ($data =~ s!(TI|AU|JN):(.*)?!!s) {
    push @{$sections{$1}}, $2;
}
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That will build a hash of arrays with the headers as keys. Things you might want to change are as follows:

• With the /s modifier on the regex, . will match newlines. You may not want this (but it makes the regex much more complicated)
• You may not want a hash of arrays. Remove the @ and use the .= operator
• You may want to match a newline after the colon.

This all depends on your data. Personally, I'd use split in a heartbeat.