a perl question

[S Mathias]

cat asdf.txt
bla-bla
bla-bla
bla[XYZ]
importantthing
another important thing
[/XYZ]
bla-bla
bla-bla
[XYZ]
yet another thing
hello!
[/XYZ]
bla-bla
etc.
$ SOMEPERLMAGIC asdf.txt > output.txt
$ cat output.txt
importantthing
another important thing
yet another thing
hello!


how can i sovle this question? what is SOMEPERLMAGIC? are there any perl gurus, that have a little spare time?

Thank you! :\