> got stuck on thinking how to open summary.txt file to read and open inout.txt to write at the same time.

This is very simple: you open a filehandle in read mode and another in write mode. You use the diamond operator <> too much: it is handy but also magic, infact instead of print "Enter the name of the file: "; my $base_dir = <>; you better my $base_dir = <STDIN>; ( $userinput is a better name than $base_dir anyway..) so start out with plain things:

# never forget the following!!!
use strict;
use warnings;
#  use diagnostics; # this can be skipped but is useful when starting

# grab user input..
# choose a file name for the out file..

...


open my $readhandle, '<', $filetoread or die "Unable to read [$filetor
+ead]!";

open my $writehandle '>', $filetowrite or die ""Unable to write [$file
+toread]!";

        # not <> but a named lexical filehandle
while (<$readhandle>){

      if ($_ =~ /somethingtosearch/){
          
         # print to the write file
         print $writehandle $_;
      }
}

# always explicitally close filehandle: Perl will normally does the ri
+ght thing but it is safer to close them anyway (good habit)
close $readhandle;
close $writehandle;

# take the file and attach to an email
...
[download]

This is the basic. After you have a working skeleton code you can add some more feature as more error control or a more elegant way to parse command line arguments aka Getopt::Simple or Getopt::Long

L*