You are on the right track, but you need something that will let you read and write to the external application.IPC::Open3 is the easiest way. IPC::Open2 works just as well but you don't have a way of getting STDERR unless you do a "$cmd 2>&1"

An IPC::Open2 example( put it in a loop if needed):

#!/usr/bin/perl
#prompts for an string to evalute 
#(line 2+2, or 5x7, 5*6 / 3 , etc) 
#sends it to the bc calculator,  
#then reads the answer, and prints. 
use IPC::Open2;
use strict;
use warnings;

my ($rd, $wr);
open2($rd, $wr, "bc");
print "Enter a string to evaluate\n";
my $prompt= <STDIN>;
print $wr "$prompt";
my $x = <$rd>;
print $x;
close($rd);
close($wr);
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A simple IPC::Open3 example

#!/usr/bin/perl
use warnings;
use strict;
use IPC::Open3;

#interface to "bc" calculator 
#my $pid = open3(\*WRITE, \*READ, \*ERROR,"bc"); 
my $pid = open3(\*WRITE, \*READ,0,"bc");
            #if \*ERROR is false, STDERR is sent to STDOUT  
while(1){
print "Enter expression for bc, i.e. 2 + 2\n";
chomp(my $query = <STDIN>);

#send query to bc 
print WRITE "$query\n";

select(undef,undef,undef,2);

#get the answer from bc 
chomp(my $answer = <READ>);

print "$query = $answer\n";
}

waitpid($pid, 1);
# It is important to waitpid on your child process,   
# otherwise zombies could be created.
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A more complex IPC::Open3 example:

#!/usr/bin/perl
use warnings;
use strict;
use IPC::Open3;
use IO::Select;

#interface to "bc" calculator 
my $pid = open3(\*WRITE, \*READ,\*ERROR,"bc");

my $sel = new IO::Select();

$sel->add(\*READ);
$sel->add(\*ERROR);

my($error,$answer)=('','');

while(1){

 print "Enter expression for bc, i.e. 2 + 2\n";
 chomp(my $query = <STDIN>);

 #send query to bc 
 print WRITE "$query\n";

 foreach my $h ($sel->can_read)
 {
   my $buf = '';
   if ($h eq \*ERROR)
   {
     sysread(ERROR,$buf,4096);
     if($buf){print "ERROR-> $buf\n"}
   }
   else
   {
     sysread(READ,$buf,4096);
     if($buf){print "$query = $buf\n"}
   }
 }
}

waitpid($pid, 1);
# It is important to waitpid on your child process,   
# otherwise zombies could be created.
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