open IN1,"cat $DATA_DIR/$INFILE_NAME |" or die "Can't open $INFILE_NAME: $!\n";

##</code><code>##

open my $in1FH, q{<}, $DATA_DIR/$INFILE_NAME
   or die qq{Can't open $INFILE_NAME: $!\n};

##</code><code>##

my @numbers = ();
while( <$in1FH> )
{
    chomp;
    push @numbers, $_;
}

##</code><code>##

chomp( my @numbers = <$in1FH> );

##</code><code>##

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};
}