If you don't need to preserve the output order in Result_file.txt you can reduce the runtime to a single pass over Input_file.dat: 
# if @tag contains simple words 
my $re = join '|', @tag;
# if they can be more complicated:
# my $re = join '|', map { "(?:$_)" } @tag;

open my $out, '+>', "Result_file.txt" 
    or die "Can't open file Result_file.txt for writing: $!";
open my $in, '<', 'Input_file.dat' 
    or die "Can't read Input_file.dat: $!";

while(<$in>){
    print $out $_ if m/$re/o;
}
close $in;
close $out;

[download]

If you use perl 5.10.0, the match against many (constant) alternatives is blazingly fast due to the trie optimizations, demerphq++

In reply to Re: Better solution to the code by moritz
in thread Better solution to the code by Anonymous Monk

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