my @filesToRead = ( populate this list somehow );
...
foreach my $file ( @filesToRead )
{
    open my $in2FH, q{<}, $file
       or die qq{Can't open $file: $!\n};
    local $/ = qq{</DU>\n};
    while( <$in2FH> )
    {
        ...
    }
    close $in2FH or die qq{Can't close $file: $!\n};
}