my $in_file = 'cn.txt';
open my $in_fh, '<', $in_file or die "Could not open file $in_file: $!
+";

my $out_file = 'liboutput.txt';
open my $out_fh, '>', $out_file or die "Could not open file $out_file:
+ $!";

while ( my $line = <$in_fh> ) {
    print {$out_fh} $line if $line =~ /\|LIB\|/;
}

close $in_fh  or die "Could not close file $in_file: $!";
close $out_fh or die "Could not close file $out_file: $!";