Well, as for the search-and-replace, this should do it:

while ( <DATA> ) {
    s{^:dir=.*?([^/]*)$}{:dir=../mainDir/sudir/$1} ;
    print ;
}
[download]

Also, since it sounds like your trying to replace all instances within a file, you might be able to save yourself a little bit of work with Tie::File.

_______________
DamnDirtyApe

Those who know that they are profound strive for clarity. Those who
would like to seem profound to the crowd strive for obscurity.
            --Friedrich Nietzsche