# 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;