This can be done from the command line:

perl -p -i.bak -e 's:CLASSATTRIBUTES:classattributes:g' *
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...though I'm not sure whether that's an option for you or not.

Update: Ah yes, please read the excellent comments below. If you're doing this from within a script, this works:

my @files = qw( foobar zoot );

for my $file ( @files ) {
    ## Read the contents of the file
    open FILE, "<$file";
    my $contents; do { local $/; $contents = <FILE>; };
    close FILE;

    ## Make your changes
    $contents =~ s:CLASSATTRIBUTES:classattributes:g;

    ## Write the modified contents back to the file
    open FILE, ">$file";
    print FILE $contents;
    close FILE;
}
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_______________
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