I'd try to use glob in scalar context:

while (my $matchingFiles = glob("*.rtsd001")) {
   ...
}
[download]

Though that doesn't seem to your real problem - in your current version you already have only one filename in memory each time.

I'd rather think your problem is @fileContents, which can hold the contents of an entire file. Instead of reading a file, selecting lines, storing those, and printing the result, you can just print them line by line:

               while (<UNMODIFIED>) {
                    /STRUC20/ and print MODIFIED $_;
               }
[download]

Thanks for the help. I gave your code a try but it is only printing the line STRUC20. What I was looking to do is print all lines in the file after STRUC20. And also all files online contain the line STRUC20 once and they will all contain that line.
Thanks again for all the help.

Example file contents of file1.rtsd001

date time
blah blah
test test
STRUC20  #Code should look for this line and print everything after it
+.
need lines
need lines
print these lines
[download]

What I was looking to do is print all lines in the file after STRUC20

The flip-flop operator (aka the range operator .. in scalar context) might come in handy for that:

while (<UNMODIFIED>) {
    print MODIFIED $_ unless 1 .. /STRUC20/;
    # i.e. do print, except for first line upto a line that matches ST
+RUC20
}
[download]

You're right, I didn't read your original code carefully enough.

You can introduce a variable that stores if it's after the STRUC20 line:

while (my $matchingFiles = glob("*.rtsd001")) {
    ...
    my $seen_struct;
    while (<UNMODIFIED>) {
        if ($seen_struct || /STRUCT20/) {
            $seen_struct = 1;
            print MODIFIED $_;
        }
    }
    ...
}
[download]

That way you don't have to store all the rest of the lines in memory, and still get the same semantics.

Update: actually this solution looks more elegant, but does nearly the same thing.

Perl 6 - links to (nearly) everything that is Perl 6.

But you might gain a bit if the files are big. In the worst case, you store the entire content in memory. You could also write the inner loop as:

use Fcntl 'SEEK_SET';
while (<UNMODIFIED>) {
    if (/STRUC20/) {
        seek MODIFIED, SEEK_SET, 0 or die;
        next;
    }
    print MODIFIED $_;
}
truncate MODIFIED, tell MODIFIED or die;
[download]

And there's no need to use /bin/mv to move the temp. file. rename will do.

Of course, the MOST efficient way doesn't involve Perl at all.

1. use strict; use warnings;
2. protect against pathologic filenames and use the 3 argument form of
   open(UNMODIFIED,"<","$processedDirPath/$matchingFiles")
   (good style, but unlikely to be the problem)
3. my bet: add a size check like

   -f "$processedDirPath/$matchingFiles" and -s _ < 40960 and 
       do{warn "size or type problem with $processedDirPath/$matchingFile
   +s\n"; next};
   [download]

4. idle curiousity: there won't ever occur further STRUC20 alone or as part of something else after the first occurance?

Update:

• what do you mean with print DATA!? These changes don't touch your copying loop at all.
• anyway, the size testing goes before your open() calls, that is: in the outer loop.
• btw, consider rename instead of mv.

Thanks for the help. I gave your code a try but it is only printing the line DATA. What I was looking to do is print all lines in the file after STRUC20. And also all files online contain the line STRUC20 once and they will all contain that line.
Thanks again for all the help.

Example file contents of file1.rtsd001

date time
blah blah
test test
STRUC20  #Code should look for this line and print everything after it
+.
need lines
need lines
print these lines
[download]

Where's your state variable?

    my $printing = 0;
    while (<UNMODIFIED>) {
        if ($printing) {
            print MODIFIED $_;
            next;
        }
        $printing++ if /STRUC20/;
    }
[download]

Also, you don't need the /bin/mv, just use unlink and rename.

Why the next?

Because additional regular expression comparisons are pointless.