This must be a very old book you are reading.

Nowadays it is customary to use the more secure 3 argument variant of open with a lexical filehandle. Also, you do not have to double quote your variables, unless you want to interpolate them into a string.

open my $READFILE, '<', $infile or die "Can not open $infile for parsi
+ng: $!" ;
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and

open my $WRITEFILE, '>', $outfile or die "Can not open new replacement
+ file: $outfile. $!";
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Your loop will then be written as follows:

while (<$READFILE>) {
     # do something to $_
     print $WRITEFILE $_ ;
}
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