# 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, '<', $filetoread or die "Unable to read [$filetoread]!";

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

        # 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 right thing but it is safer to close them anyway (good habit)
close $readhandle;
close $writehandle;

# take the file and attach to an email
...