use strict;     #These two lines are almost-always
use warnings;   #a good idea

print "Enter your data.\n";
print "Type .. at the beginning of a line to quit.\n";
open my $output_file,'>','output_file.txt' 
     or die "We got trouble here!\n";
LOOP:
while (<STDIN>) {
   my $data = $_;
   chomp $data;
   last LOOP if ($data =~ m/^\.\./);
   print $output_file "$data\n";
}
close $output_file;
exit;