There is nothing to it. Two files, two filehandles. For example:

my $filename1 = "f1.txt";
my $filename2 = "f2.txt";

open F1, "< $filename1" or die "read $filename1";
open F2, "< $filename2" or die "read $filename2";

# assuming the files are exactly parallel, line for line:

while (<F1>) {
  my $line1 = $_;
  my $line2 = <F2>;

  # etc.
}

close F1;
close F2;
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open(FILE_A, "< file_a.txt") or die "Cannot open file_a: $!\n";
open(FILE_B, "< file_b.txt") or die "Cannot open file_b: $!\n";

while (1) {
    my $line_a = <FILE_A>;
    my $line_b = <FILE_B>;

    last if not defined $line_a or not defined $line_b;
    
    # Processing of $line_a and $line_b...
}
close(FILE_A);
close(FILE_B);
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Arjen

open (FILE1, "file1.txt") or die "Can't read file1: $!";
open (FILE2, "file2.txt") or die "Can't read file2: $!";

#.... do whatever you need to each file, accessing them by the 
# appropriate filehandle.
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If the former, no single CPU computer can do it, but some multiprocessor machines may be able to, provided their operating system supports it.

If the later (more probable), then there is no problem:

open(FILE1,"</some/file/somewhere") || die "Could not open FILE1: $!\n
+";
open(FILE2,"</some/other/file") || die "Could not open FILE2: $!\n";
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Or, if you open the files for writing, you can write to them like this:

print FILE1 "something to be written to FILE1\n";
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