With open (FH, "<$file1") or die "blahblah"; there is a possible security issue because maybe the variable $file1 might contain something that would cause problems. What goes in to the open() function is the interpolated string. You learned right, always use the < or > etc. In your example, if $file1 was ">ImportantFile", the open() will fail because the argument would evaluate to "<>ImportantFile" and open() won't like that filename! If you forgot the "<", then then open to ">ImportantFile" might succeed and it would be deleted or other various bad things could happen.

So if you use:  open (FH, '<', $file1) || die; putting the file mode explicitly there is a "small" thing that could save you big problems later.

If you are writing small one or two page programs, using a bare word like FH is no big deal. However, be aware that Perl like C (you have do it this way in C), can use lexical variables for filehandles. So you can open($infile, '<', $somefile)... and pass $infile to a subroutine just like any other Perl variable.

As far as $! in "die" messages, you might or might not want to put that there. Part of this depends upon how descriptive your part of the "die" message is! Ok, try some code: 

#!/usr/bin/perl -w
use strict;

open(FH, '<', "bad") || die "your textXXX OS says: $!\n";

while (<FH>){} # prevents warning FH only used once

[download]
See what $! has to say. My OS prints, "your textXXX OS says: No such file or directory". I figure that "your textXXX" is way more important. Something like "can't open Budget.csv" is way more to the point than "No such file or directory"- most of the time the OS text is meaningless for the average user. Also notice what adding the trailing "\n" to the die message does.

Update: This thread has morphed into something else from the OP's original question. But I figure it is ok to comment on some of the comments to the comments!

In reply to Re^3: Reading two text files parallelly... by Marshall
in thread Reading two text files parallelly... by biswanath_c

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