My first thought is to use IPC:Open3

A simple example:

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

#interface to "units" calculator
#my $pid = open3(\*WRITE, \*READ, \*ERROR,"units -t"); #terse output

my $pid = open3(\*WRITE, \*READ, 0 , "units -t");
            #if \*ERROR is set to 0, STDERR is sent to STDOUT 

#chomp(my $prompt = <READ>);
#print "$prompt\n";

foreach ( '100 yards', '1 mile', '1 kilometer', '1 parsec', '1 foobar'
+){  

print WRITE "$_\n meters \n";

chomp(my $answer = <READ>);
print "$_ = $answer meters\n";

}


__END__
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