I'm going to comment your script one line at a time - it really is an excellent opportunity to explain some good bits of Perl (presumably, that's just what the author of your book intended.) My comments are prefixed with a '###'.

#!usr/bin/env perl

### I suspect that you're retyping your code, since the
### above would fail with a "Bad interpreter" error: there
### is no such thing as "usr/bin/env" (note absence of
### initial '/'.) As well, the 'env' trick isn't usually
### necessary unless you're moving your script between
### radically different systems where Perl is located
### in different places; usually, "#!/usr/bin/perl" will do.
### In addition, you should always - yes, *always* - enable
### warnings (i.e., let the computer do your troubleshooting 
### for you) - so that shebang line should also have a ' -w'
### added to its end.

open (test, "@ARGV");

### Problem #1: you're trying to use an array (@ARGV) where
### you should be using a string containing a filename. This 
### _will_ break if @ARGV contains more than one element.
### Problem #2: Whenever you perform any system-related task
### - i.e., an operation that is external to Perl - you
### should always (yes, *always*) check the result. As a
### minor additional correction, you should elide any
### unnecessary punctuation in your Perl - and you should
### either use sentence-case or all-caps names for file
### handles. As a result, the corrected version of the above
### line should read
###
### open Test, $ARGV[0] or die "$ARGV[0]: $!\n"; 

while (<test>) {
    $_ =~ <test>;
    
    ### This line is a simple error, and should be omitted.

    chomp;
    
    ### This is unnecessary, since you're going to want
    ### the newline character when you print the output.
    ### Omit it.

    s/(th)/TH/gi;

    ### Neither grouping nor capturing are used in the
    ### above regex; therefore, the parentheses do
    ### nothing and should be omitted.

    print "$_\n";

    ### If you omit the above 'chomp', it will become
    ### unnecessary to explicitly specify '$_' or '\n'.
    ### This line then simply becomes 'print;'.

}

close (test);

### Again, the parens are unnecessary. Omit them.
[download]

As a result, the entire script should look like this:

#!/usr/bin/perl -w

open Test, $ARGV[0] or die "$ARGV[0]: $!\n";

while (<Test>) {    
    s/th/TH/gi;
    print;
}

close Test;
[download]

Conversely, you could use Perl's "diamond operator" which will do the same thing without having to explicitly open the file that you've specified on the command line:

#!/usr/bin/perl -w

while (<>) {
    s/th/TH/gi;
    print;
}
[download]

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