Have a look at open and print. This code does nothing but copy an input file to the output file to show their use. For your application, you'll want to do a lot more "stuff" between the reading and writing.

my $in_filename = 'in.txt';
my $out_filename = 'out.txt';

open my $in_fh, '<', $in_filename
    or die "Can't read '$in_filename': $!";
open my $out_fh, '>', $out_filename
    or die "Can't write '$out_filename': $!";

while ( my $in_line = <$in_fh> ) {
    print {$out_fh} $in_line;
}

close $in_fh    or die "close failed: $!";
close $out_fh   or die "Close failed: $!";
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