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: $!";