Each time Perl sees an <test> it reads another line from 'test'. There is no problem with using =~ in conjunction with your <test> construct...but it does cause another read from test. I'm not sure what you were intending or hoping the line $_ =~ <test> was supposed to do. Can you say what you were thinking?

In case you're wondering, the other read occurs in your loop control line while(<test>).

To keep it from reading twice but to still do what it looks like you're wanting to do you can use a construct like:

while (<test>) [
    chomp;
    s/(th)/TH/gi;
    print "$_\n";
}

close (test);
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Several other monks that have answered your inquiry show this and I think it is the right answer. But since I'm not sure what you intended with the line $_ =~ <test>, I'm not positive that deleting the line really results in what you were intending.

C:\>perl -MO=Deparse,-p
#!usr/bin/env perl

open  (test, "@ARGV");

while (<test>) {
    $_ =~ <test>;
    chomp;
    s/(th)/TH/gi;
    print "$_\n";
}

close (test);
^Z
open(test, "@ARGV");
while (defined(($_ = <test>))) {
    ($_ =~ <test>);
    chomp($_);
    s/(th)/TH/gi;
    print("$_\n");
}
close(test);
- syntax OK
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